The effect of thermal treatment over a wide range of temperature (130–280°C) on the crystallization behavior of nylon 6 was studied by using DSC, FTIR, and polarized light microscope equipped with a hot stage. The crystallization and the subsequent melting behavior of the nylon 6 samples treated at different temperatures (Ts) were classified into four types. When Ts was higher than 236°C or lower than 213°C, the crystallization behavior of nylon 6 was insensitive to the variation of Ts. When Ts was in the range of 213–235°C, the crystallization behavior was sensitive to the change of Ts. The polarized light microscopic experiments have demonstrated that a large amount of tiny ordered nylon 6 segments/cluster persisted when nylon 6 film are heated to 231°C. Consequently, the fastest crystallization speed was observed. As Ts was between 214 and 223°C, both the Tm and the ΔHm were higher than those of the nylon 6 samples treated at other temperature. The polarized light microscopic investigations have also demonstrated that molten nylon 6 crystallizes by using the un-molten nylon 6 crystals as nucleation center at 220°C. Crystallization at higher temperature produces nylon 6 with thicker crystalline lamella. The above results are helpful for rational design of thermal treatment procedure to obtain nylon 6 with different crystalline features.