Objective: To compare the cleavage activity of ribozymes directed against 2 sites of PDGF receptorβ subunit cDNA gene in vitro. Methods: The 608 bp fragment of PDGF receptor β subunit cDNA was cloned into T-vector under the control of T7 promoter, named pPDGFR-β. Two ribozymes were designed to cleave the CUU sequence at codon 45 and codon 252 of PDGF receptor β subunit mRNA respectively. These 2 hamate the plasmids of pRZ1 and pRZ2. The pPDGFR-β, pRZ1 and pRZ2 -were linearized and then transcribed with T7 promoter in vitro. Results: The RZ1 showed high cleavage activity in vitro, but the RZ2 showed no cleavage activity under the same condition. Conclusion: The cleavage site selection is an important factor influencing the cleavage activities of ribozymes.