Abstract: Microstructure and performance of inter-critical region (ICHAZ) in which peak temperature is between A_c1 and A_c3 in the heat-affected zone of HQ130 steel are researched by means of thermosimulation test, micro-image analysis, SEM and TEM etc. For the problem of toughness decrease in the ICHAZ (T_p=800℃), the effects of M-A constituent and carbide aggregation on the brittleness were studied emphatically. The research results indicate that the microstructure in the ICHAZ of HQ130 steel is the mixture of lath martensite (ML), granular banite (Bg) and sorbite (S) whose grains are small but not homogeneous. Cause of brittleness in the ICHAZ is related to produce M-A constituent in the local zone and the aggregation of lamellar carbide. The weld heat input has important effect on volume content (V_f) of M-A constituent but only little effect on long and thin M-A constituent content (SV_f). By controlling weld heat input (E), the aggregation of carbide and M-A constituent can be avoid or decreased, and the effective diameter of M-A constituent is limited in the rang of d_M-A<1.0 μm so the toughness in the ICHAZ can be assured.