Kappaphycus species, cultivated widely for carrageenan production, are tropical marine red algae, which are sensitive to high temperature. Due to the increase in global mean air temperature, all kinds of metabolic processes in numerous marine algae are being affected by high-temperature stress. Nitrate reductase (NR) is one of the crucial enzymes in the nitrogen metabolic process. As one of the members of the protective enzyme system, peroxidase (POD) can decompose hydrogen peroxide generated by various metabolic pathways in the plant. Chlorophyll and phycoerythin, correlated to the photosynthetic process, are important pigments in red algae. Chlorophyll fluorescence technique provides a quick, convincing, and non-destructive means for assessing the efficiency of photochemical conversion and it has become an increasingly powerful tool widely used in the study of photosynthesis. Actual photochemical efficiency (yield, Y) and the relative electron transport rate (ETR) are commonly used as chlorophyll fluorescence parameters correlated to the photosynthetic processes. In this study, the effect of high temperature on the above-mentioned physiological characteristics of K. alvarezii and K. striatum were examined by means of stationary culture in the laboratory. The activities of NR and POD, and the contents of chlorophyll a and phycoerythrin in the two algae were determined at 32℃, 35℃ and 40℃, with 25℃ as a control. The changes of Y and ETR in the two algae under high temperature were detected. The results showed that the activities of NR and POD were affected significantly by high temperature (P < 0.01). NRs in the two algae showed minimum activities at 35℃, while maximum activities at 32℃. NR activities in K. alvarezii were 4.1 times stronger than those in K. striatum at 32℃. However, POD activities varied in the two algae. They were highest at 35℃, and lowest at 25℃ in K. striatum. In contrast, in K. alvarezii they were lowest at 35℃ and highest at 25℃. The activities of POD in K. striatum were 63.9 times stronger than those in K. alvarezii at 35℃. When the temperature rose from 25℃ to 40℃, the content of chlorophyll a in K. striatum gradually decreased while the content of phycoerythrin first decreased then increased and finally decreased again with the maximum obtained at 35℃. Conversely, the contents of both chlorophyll a and phycoerythrin in K. alvarezii were highest at 25℃ and lowest at 35℃ and 32℃, respectively. The Y and ETR in the two algae obviously decreased with the increase of temperature from 32℃ to 40℃. The degree of decrease of the two parameters was in direct proportion to the increase of temperature. The photosynthetic activities in the two algae intensified with short-term high temperature. However, with the increase of exposure time, the photosynthetic activities in K. striatum decreased nonsignificantly while those in K. alvarezii fell significantly (P < 0.01). In conclusion, all detected indexes indicated that high-temperatures above 32℃ generated stress in the two algae. The stress was weak below 35℃ and strong over 40℃ in K. striatum. Stress increased considerably over 32℃ in K. alvarezii. K. striatum was more resistant to high-temperature stress than K. alvarezii.