The crystallization behavior of high-density polyethylene (HDPE) on highly oriented isotactic or/and syndiotactic polypropylene (iPP or/and sPP) substrates at various conditions was studied by means of transmission electron microscopy and electron diffraction. The results indicate that HDPE crystals always grow epitaxially on the iPP substrates under the conditions performed, while on the oriented sPP substrate epitaxially grown HDPE can only be observed when quenching the HDPE-melt directly to room temperature. The reason for this phenomenon results from the thinner lamellae of sPP compared to iPP, and consequently smaller critical nucleus size of HDPE is required for the occurrence of its epitaxial crystallization on the sPP substrate. It is further found that the melt-drawn oriented polymer films are ideal substrates for conducting comparative studies of polymer hetero and graphoepitaxy. By comparing the hetero and graphoepitaxies of HDPE-iPP and HDPE-sPP systems, it is concluded that the heteroepitaxial interaction between polymers is stronger than their graphoepitaxial counterpart. Therefore, heteroepitaxy of polymers takes place prior to their graphoepitaxy