The melting behaviors of polyethylene and poly(vinylidene fluoride) have been experimentally examined by thermal analysis, optical microscopy, and atomic force microscopy. The melting velocity is found to show a nonlinear dependence on the degree of effective superheating. The dependence clearly suggests the existence of an activation barrier, though crystal melting is generally supposed to proceed without any barriers. The behavior has been reproduced by a Monte Carlo simulation of the entropic barrier of pinning proposed by Sadler and Gilmer for chain-folded crystallization of polymers.