Strong dependence of the crystal orientation, morphology, and melting temperature ($T_m$) on the substrate is observed in the semicrystalline polyethylene thin films. The $T_m$ decreases with the film thickness decrease when the film is thinner than a certain critical thickness, and the magnitude of the depression increases with increasing surface interaction. We attribute the large $T_m$ depression to the decrease in the overall free energy on melting, which is caused by the substrate attraction force to the chains that competes against the interchain force which drives the chains to crystallization.