Cottons (Gossypium spp.) are of tropical origin, and are the most important textile fibre crops in the world. The most widely distributed commercial cotton species worldwide is upland cotton (G. hirsutum) which has superior yield, followed by pima cotton (G. barbadense) which has superior fibre properties. However, both lower pima cotton yields and the need for a longer growing season restrict the production of this higher quality cotton which has particularly promising for cotton production. From now on, breeding programmes for produce commercially viable genotypes have failed through incorporating the desirable fibre traits of pima cotton into upland cotton. In china, pima cotton production uniquely concentrates in Xinjiang Province. Here, compared with upland cotton, lower of photosynthate production of pima cotton was one of mainly reasons for its lower yield. The principal aim of this study was to reveal underlying mechanism attributed to the difference of photosynthetic capacity between pima cotton and upland cotton and to explore the possibilities of improving the photosynthate production of pima cotton. The experiment was conducted in Xinjiang Province, northwest China. Two typical cotton cultivars, pima cotton Xinhai 22 and upland cotton Xinluzao 13 were selected as the experimental materials. We measured diurnal time course of net photosynthetic rate (P_N), stomatal conductance (G_s) and leaf temperature. On the day of measurement, leaves of Xinluzao 13 were brought to the horizontal position and retained in that position by nylon strips tied to a metal frame in order to remove the difference of incident leaf sunlight because Xinluzao 13 has diaheliotropic leaf movement but Xinhai 22 has not. We also measured the P_N-C_i (intercellular CO₂ concentration) response curve, P_N-PPFD (photosynthetic photon flux density) response curve, chlorophyll content, leaf area, leaf mass per area and above-ground biomass, as well as leaf anatomy. The results showed that if leaves of Xinluzao 13 were restrained to the horizontal position throughout the day, the leaf temperature in Xinhai 22 was higher than that in Xinluzao 13. In the early morning and late afternoon where sunlight intensity was low, there was no difference in P_N and G_s between Xinhai 22 and Xinluzao 13. From 12:00 h to 16:00 h, both P_N and G_s in Xinhai 22 were lower than that in Xinluzao 13. A significantly positive line regression was found between P_N and G_s. Both cotton cultivars had similar P_N-C_i response curves. P_max (light-saturated photosynthetic rate) and AQY (apparent quantum yield) were derived from P_N-PPFD response curve. Compared with Xinluzao 13, Xinhai 22 had lower P_max, whereas both cotton cultivars had similar AQY. Both leaf thickness and palisade tissue length in Xinhai 22 were thinner than that in Xinluzao 13. Xinhai 22 had higher chlorophyll content and leaf area than Xinluzao 13. Nevertheless, compared with Xinhai 22, both leaf mass per area and above-ground biomass were higher in Xinluzao 13. All results suggested that pima cotton had lower actual photosynthetic capacity than upland cotton in the field even if diaheliotropic leaf movement of upland cotton resulting in high incident leaf sunlight was not considered. However, both cotton cultivars inherit similar photosynthetic potential. Stomatal conductance was an important reason for difference of actual photosynthetic capacity between pima cotton and upland cotton. Furthermore, thinner palisade tissue in pima cotton leaves limited the expression of photosynthetic potential in the field.