The high-tenacity nylon 6 filaments were heat-treated in dimethylformamide(DMF) and silicone oil at various temperatures(40-120$^{circ}C$) under the unrestrained and constant-length conditions for three hours. And some measurements about structure and physical properties of the samples were done, and the values taken from the measurements were investigated for the changes and the correlations of structure and physical properties to be inquired into. The results are as follows, 1) The increase of crystallinity in DMF begins at lower than 40$^{circ}C$ and the increasing tendency becomes more pronounced as the treating temperature increases, but the increase of crystallinity in silicone oil begins at higher than 100$^{circ}C$. The increasing tendency of the crystallinity is more remarkable under the unrestrained condition than under constantlength condition. 2) The birefringence decreases gradually as the treating temperature increases. The decreasing tendency of the birefringency in DMF is more prominent than in silicone oil. 3) The decrease ratio of length and the increase ratio of diamerter gradually increase as the treating temperature increases, and the percentages of dimensional changes are higher under the unrestained condition than under the constant-length condition. The initial modulus decreases and the elongation increases as the treating temperature increases. And the extents of their changes are more remarkable under the unrestrained condition than under the constant-length condition, especially in DMF than in silicone oil. 5) The tensile strength of samples treated under the constant-length condition increases gradually as the treating temperature increases but has maximum value under the unrestrained condition.