Even though self-nucleation is considered to be the ideal case for polymer nucleation, it has rarely been used to enhance polymer crystallization in practical processing techniques. Inspired by self-nucleation theory and by utilizing the large difference in melting points of various poly(L-lactide) (PLLA) resins, we introduced high-melting-point PLLA (hPLLA) crystallites into a low-melting-point PLLA (lPLLA) matrix via melt blending at a processing temperature between the two melting points of the selected PLLA resins. The hPLLA crystallites turn out to be efficient nucleating agents (NAs) for lPLLA and high crystallinity (>40%) PLLA samples with a greatly accelerated crystallization rate can be easily obtained. The results of non-isothermal crystallization show that the crystallization process is remarkably accelerated with a small amount (0.1 wt%) of hPLLA. With a further increase of hPLLA content, the crystallization temperature of the blends continues to shift to higher temperature. This crystallization promoting effect results from the excellent nucleation ability of the hPLLA crystallites, as revealed by in situ optical microscopy observation. Furthermore, an incredibly high nucleation efficiency of 103.0% (exceeding 100%) was obtained for the PLLA sample with 5.0 wt% hPLLA. The nucleation mechanism for hPLLA was studied systematically. It was found that the lPLLA matrix and hPLLA crystallites possess an absolutely identical crystal structure of α-form crystals and an excellent interfacial interaction between the nucleating agent, i.e., hPLLA crystallite, and the lPLLA matrix is achieved, resulting in the reduction of the energy barrier for heterogeneous nucleation and acceleration of crystallization kinetics. Therefore, by using high-melting-point polymer crystals as NAs, the crystallization rate of their low-melting-point polymer matrix can be improved largely, thus providing a simple way to obtain high crystallinity products for semicrystalline polymers with very low crystallization rates.