Small Brown Planthopper (SBPH) Laodelphax striatellus (Fallen) and Brown Planthopper (BPH) Nilaparvata lugens (Stål) have been the major insect pests in Chinese rice ecosystem at different times since 1960s, but simultaneously caused serious damage at the heading stage in the Yangtze Delta area in the recent years. To understand the ecological mechanisms of simultaneous outbreaks of the two rice planthopper species in the area, the density related intra- and inter-specific effects for SBPH and BPH on their main biological parameters, including nymphal development duration, emergence rate, longevity and fecundity of female adult, were studied in the laboratory using indica hybrid Shan You 63 and japonica Xiu Shui 11 as host plant varieties. The results showed that there was significant intraspecific competition, as well as interspecific facilitation, for both planthopper species. Intraspecific competition resulted in prolonged nymphal development duration, lower emergence rate, shorter longevity and lower fecundity of female adult as initial density was increased. In contrast, the interspecific facilitation resulted in shorter nymphal development duration, higher emergence rate, prolonged longevity and higher fecundity of female adult when the two species were coexisted. Both intra- and inter -specific effects were significantly interacted with planthopper species and host variety. The density related intra- and inter-specific effects on L. striatellus is higher than those on N. lugens. The intra-specific competition on a relative suitable host variety was higher than that on the other variety, but the inter-specific facilitation on the relative unsuitable host variety was higher than that on the other host variety. Findings are discussed in relation to underlying mechanisms and evolutionary significance, as well as the possible interaction between interspecific facilitation and simultaneous outbreak of SBPH and BPH.