Microstructure and formation characteristic of laser welded lap joints of TC4 alloy were investigated, and a parameter named join rate ψ was introduced to evaluate the welding effect of lap welding. The results indicated: the width of weld bead increases with increasing laser power or decreasing welding velocity, the top width of weld bead is larger than the bottom width and the minimum is the middle layer. And the join rate decreases as plates space or energy input increases. Microstructure of weld bead consisted of α′ martensite, and the martensite in the top portion is more concentrate than that in the bottom portion. There ate a small amount of martensite in HAZ, and distributed in gradient. For the strengthen effect of martensite, the microhardness reduces from the center of joints to base metal in cross direction, and the microhardness is higher in top weld bead