In a case of welding of plastic substrates, appropriate clamping pressure is needed. However, melt plastic is easily transformed by the excessive pressing force such as by comprehensive hot-plate welding. More defects such as decrease of thickness or cutoff can also happen. The authors have applied the surface―cooling laser welding method for fluoroplastic sheets. A laser welding experiment for overlapped fluoroplastic PFA sheets was conducted to examine the influence of clamping pressure during laser irradiation on tensile strength of weldments. Furthermore, an evaluation of pressure at the interface of two sheets by numerical analysis was investigated. The result of experiments showed that bubbling had occurred within the melt zone due to increased contact thermal resistance between the material and the heat sink in the case of no or negligible pressure, resulting in a decrease in tensile strength. Secondly it was found that contact heat resistance decreased and sufficient heat release was possible when the clamping pressure was 0.05 MPa, but that tensile strength was not affected when the clamping pressure was increased beyond that point. Furthermore, a comparison of the contact pressure at the welding interface was conducted by numerical analysis. The result showed a pressure dispersion loss due to deformation of the welded part in the case of hot plate welding, while thermal expansion caused internal contact pressure in the case of laser welding. As expected, this result was a factor in realizing sufficient welding without external deformation, even at a low clamping pressure.