The compression induced by the assembly of proton exchange membrane (PEM) fuel cells causes partial deformation of the gas-diffusion layers (GDLs) and, consequently, influences the performance of PEM fuel cells. In order to investigate how assembly pressure affects electric efficiency of PEMFC, performance of PEMFC experiments with a miniature self-humidifying, breathing PEMFC stack are conducted under different clamping pressures. The polarization and power efficiency curves of PEMFC under different clamping pressures show that the best performance can be obtained at the allowable lower limit of working parameters. The research shows that the effect of assembly clamping pressure is significant, and the low clamping pressure is beneficial to improve the performance of the PEMFC stack while keeping sealing. The experimental results indicate that a clamping pressure of 1MPa improves the fuel cell performance in this paper.