A study was made to examine the effects of heat-treatment on the microstructures of Cu-bearing HSLA steels. The details of microstructures were studied by using optical microscopy(OM), transmission electron microscopy(TEM) and hardness test. The as-rolled microstructure of HSLA-A consists of ferrite (${alpha}$) whereas that of HSLA-B consists of needle-shaped phase. The difference in microstructure can be ascribed to the different amount of Ni, Mn, Mo, Cu which affect the hardenability. The austenite grain size is very small up to $1000^{circ}C$ owing to the Nb-precipitates. As the austenitizing temperature increases above $1000^{circ}C$, the austenite grain grows rapidly. There are two hardness peaks in the hardness versus temperature curve, at $500^{circ}C$ and at $675^{circ}C$ (HSLA-A), $725^{circ}C$ (HSLA-B). The peak at $500^{circ}C$ result from the formation of Cu-precipitates and second hardness peak is created due to the formation of M-A constituents. The hardriess decrease in HSLA-B steel with ageing temperature is small because of the higher amounts of Cu than HSLA-A steel. The fine, round ${varepsilon}$-Cu precipitates grow with ageing temperature and finally transform into rod shape.