Monomer casting polyamide 6 (MCPA6)/polystyrene (PS)/montmorillonite (MMT) ternary composites were prepared via successive in situ polymerization. The effects of Na+-MMT and OMMT on morphologies of MCPA6/PS blends have been studied by scanning electron microscopy (SEM) and transmission electronic microscopy (TEM). Incorporation of certain amount of Na+-MMT results in the diameter of PA6 dispersed microspheres in PS matrix increased significantly from several to dozens of micrometers. It was interesting to find that all Na+-MMT was dispersed only in the MCPA6 microspheres. However, a different morphology was observed in MCPA6/PS blend with incorporation of OMMT, especially when the content of OMMT a parts per thousand yen1 wt%. That is, the morphologies of the ternary blends change from dispersed PA6 microspheres/PS matrix to dispersed PA6 network with inclusion of local PA6 microspheres/PS matrix. It was surprising that OMMT dispersed in PA6 microspheres at low OMMT loading (i.e., 0.5 wt%), while OMMT was dispersed in PA6 network phase, not in PA6 microspheres phase at high OMMT loading (i.e., a parts per thousand yen1 wt%). The incorporation of a various amount of MMT affected the crystallization behavior of PA6 phase in the MCPA6/PS blends by the results of XRD and DSC. The MMT layers leaded to the co-existence of alpha-crystalline form and gamma-crystalline form of PA6. Furthermore, the more the MMT contents, the stronger the intensity of the gamma-peak. Another interesting phenomenon was that there was a remarkable difference in molecular weight of PA6 microspheres phase and PA6 network phase in MCPA6/PS/OMMT (2 wt%) ternary composites from GPC results.