This paper summarizes a sytematic study aimed at revealling the sag resistance of a new low carbon steel 35SiMnB and the effect of intercriticasl treatment on it by using of tension and compression hysteresis loop area (HLA) and Bauschinger effect (BE), also in the end contrast with 60Si2Mn. The results show that the sag resistance manifested by HLA of 35SiMnB is lower than that of 60Si2Mn, but the static load loss and cycle stress relaxation of 35siMnB are almost the same as 60Si2Mn. So it can be thought that the sag resistance of 35SiMnB as satisfactory for the application to leaf springs. The physical significace of HLA was approached in the paper. The method measured HLA and conditions being suitable for using HLA to manifeste sag resistance were suggested.