Abstract: A three-dimensional finite element analysis thermal model for laser transmission welding technique of polycarbonate(PC) was developed. Power distribution of moving heat source was implemented into DFLUX subroutine, written in FORTRAN programming language to simulate the temperature distribution during the laser transmission welding process. The effect of process parameters on the weld strength and weld width was analyzed. Data gained by numerical simulation was compared to data obtained by laser transmission welding experiments, and good agreement was found between the two trends. It was observed that for a particular value of the laser power, good bonding between two thermoplastic materials is a function of laser scanning speed. Too high speed will not produce any significant increase in temperature at the two material interfaces to have a good chemical bonding. On the other hand, too slow speed will cause excessive increase in temperature resulting in burnout condition for polycarbonate. For the ranges of parameters investigated in the current study, it was observed that for a laser heat flux of 40 W, good bonding occurs for a laser scanning of 40 mm/s. This shows that the finite element analysis can be used for optimization of process parameters.