In this study,we discussed the effect of seed size and altitude on the C,N,P contents of 53 Compositae plant seeds on the northeastern Qinghai-Tibet Plateau,and provided theoretical evidence for discussing the response of plants to environmental change.Seed size was defined by the average of three samples with hundred seeds,and element content of seed was measured with K₂Cr₂O₇ external heating method and flow injection analyzer.The results showed that:(1)There was significant difference in seed size of 53 Compositae plants,and a trend that seed size decreased with increasing altitude;(2)There was no significant difference in the C,N,P contents of seeds and their ratio among altitudes.However,there was a trend that the C,N content of seeds increased and the P content of seeds decreased with increasing altitude;(3)The C contents of seed did not change with seed size variation.While the N and P contents of seeds had significant negative correlation with seed size,and C/N,C/P and N/P ratios have significant positive correlations with seed size.The seeds with more relative contents of N and P are going with smaller seed size.This study suggested that the more accumulation of C and N in seeds could offer more energy and synthesis of proteins in the earlier stages of plants,and then would improve the ability of seedlings to resist harsh environment and enhanced the survival rate;the more contents of N and P in smaller seeds could offer enough proteins and ribosomes for the earlier growth of seedlings and help them to grow fast,and then would overcome the disadvantages due to smaller size.Thus,it is an adaptive selection that contents of C,N and P in seeds changed with seed size variation and altitude gradients.