The effect of the laser welding parameters such as laser power, welding speed on weld strength and microstructure is investigated. A 300 W YAG laser with wavelength of 1064 nm is used to weld the transparent and absorbing polycarbonate (PC) in a lap weld configuration. The force at break of the lap welds is assessed on the universal testing machine. The weld fracture surfaces and weld cross-sections are visualized using polarized light microscopy to qualitatively assess the weld quality. The result shows that for transparent PC plastic thickness between 1 mm and 3.5 mm, the transmittance does not change significantly and the part thickness usually has little influence on transmittance. The weld strength shows a clear increase with the line energy increasing. The weld strength reaches a maximum when the line energy raises to about 1 J/mm and then starts to fall off. A peak tensile strength of 1.3 kN is obtained with transparent part thickness of 3.5 mm, laser power of 40 W and welding speed of 40 mm/s. It is concluded that in laser welding of thermoplastic material, the heat input should be strictly controlled to improve the welding strength.