The effect of[S] on strength and toughness of low alloy steel plates in which sum of[P],[N], T[O] is less than 8×10^-5 and the effect of T[O] on strength and toughness of the steel plates in which sum of[S],[P],[N] is less than 7×10^-5 were investigated. It is found that the strength of the steel plates decreases with increasing[S] content when[S] is less than 4×10^-5. When[S] varies within the range of 4×10^-5-1.2×10^-4,[S] has no significant effect on strength of the steel. The strength of the steel plates increases with increasing T[O] content when T[O] is less than 30×10^-6, but decreases with increasing T[O] when T[O] is more than 3×10^-5.The difference between the LETT in plate length direction and LETT in width direction decreases with decreasing[S] content. However, even when[S] is decreased to 9×10^-6, the difference of the LETT is still 16℃. When T[O] varies between 1.8×10^-5 and 5.2×10^-5, the low temperature impact toughness of the steel plates slowly decreases with T[O] increasing. When T[O] increases to more than 5.2×10^-5, the low temperature toughness of the steel rapidly decreases with increasing T[O] content.