Castor seed size varies greatly due to breed,has yet to see molecular biology research on castor bean size traits associated with the development.In this experiment,32 castor seeds of different sizes as research material,with U25 (55) uniform design for seed size traits RAPD-PCR reaction system and conditions were optimized.The optimal reaction system:25 μL system,10 × Buffer 2.7 μL,dNTP 2.4 μL,Mg2 + 2.4 μL,DNA 1.7 μL,primers 1.9 μL,Taq polymerase 2.0 μL,ddH2O 11.9 μL.The best reaction conditions were:94 ℃ 150 s;94 ℃ 45 s,39 ℃ 65 s,72 ℃ 90 s,35 cycles;72 ℃ 10 min;4 ℃ preservation.With SB-002 primer(nucleotide sequence(5'-3'):TGCCGAGCTG),using the best reaction system and conditions,screening a different band in the largest grain castor seeds,sequencing results were compared in the NCBI found with high yield rice genome DNA,mRNA or cDNA homology of 100%.Therefore,It can be concluded that the different band relevant to castor seed yield,laid the foundation for the relevant molecular mechanisms of castor seed size traits studied.