In the present work, we prepared nylon 6 crystals via crystallization of nylon from phosphoric acid by using the vapors of ammonium hydroxide as a precipitation regent. Both XRD and FTIR results demonstrate that the obtained nylon 6 sample exhibit characteristic peaks of nylon 6 in γ form. In addition, treatment of nylon 6 in boiling water for half an hour followed by FTIR and XRD characterization shows that the obtained nylon 6 sample is in γ form rather than in meta-stable b form. DSC characterization indicates that the nylon 6 sample exhibits two melting peaks (213 and 220 ℃) when the sample is heated at a heating rate of 10 ℃·min-1. The reason for this phenomenon is that the nylon 6 sample has different lamellar thickness. To investigate the thermal behavior of the nylon 6 sample, the sample underwent the following thermal treatment procedure. First, the sample was heated to a pre-set temperature (Ts) and kept at that temperature for an hour. Subsequently, the sample was cooled down to 100 ℃ at a cooling rate of 1 ℃·min-1, and then cooled down to room temperature at a cooling rate of 10 ℃·min-1. The treated samples were characterized by FTIR and DSC method. Experimental results show that the treated nylon 6 samples exhibit different crystalline behavior. When Ts ranges from 130 to 160 ℃, no significant changes were observed. When Ts is 170 ℃, a small fraction nylon 6 crystals is destroyed and recrystallized into thin lamellae in a form. As a result, a pre-melting peak appears in DSC result. The pre-melting peak moves to higher temperature and its peak area increases significantly upon increasing Ts from 170 to 198 ℃. When Ts amounts to 200 ℃, the pre-melting peak and the melting peaks 213 ℃ merge into one melting peak and two melting peaks are observed at 212 and 220 ℃ in the DSC results. FTIR spectra indicate that significant amount of crystalline nylon 6 in a form appears but the majority of crystalline phase of the sample is still γ phase. As Ts increases from 200 to 209 ℃, the melting peak at lower temperature moves to higher temperature with increasing its peak area. On the other hand, the melting peak at 220 ℃ decreases in intensity but does not show any peak shift. As Ts reaches 209 ℃, the two melting peaks merge into one peak and FTIR results demonstrate that nylon 6 in a form becomes dominate phase in the sample. In the whole heat-treatment process, the γ phase nylon 6 sample began to transform to α phase at the heat-treatment temperature of 170 ℃, which is far below the melting point of the original sample (221 ℃). This is different from the results reported in the literature, which state that γ phase nylon 6 will not transform to α-phase until nylon is melt.