PDMS (polydimethylsiloxane) collapse method is a simple and low cost approach for micronanochannel fabrication. However, the bonding pressure which influences the size of the final PDMS micro/nanochannels has not yet been studied. In this study, the effect of the bonding pressure on the size and maximum local stress of the PDMS micronanochannels was investigated by both experimental and numerical simulation method. The results show that when the bonding pressure is lower than 0.15 MPa the experiment results can agree well with the simulation results. The fluorescent images demonstrate that there is no blocking or leakage over the entire micro/nanochannels.