Abstract: The effect ofS on strength and toughness of low alloy steel plates in which sum ofP,N, TO is less than 8×10^-5 and the effect of TO on strength and toughness of the steel plates in which sum ofS,P,N is less than 7×10^-5 were investigated. It is found that the strength of the steel plates decreases with increasingS content whenS is less than 4×10^-5. WhenS 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 TO content when TO is less than 30×10^-6, but decreases with increasing TO when TO is more than 3×10^-5.The difference between the LETT in plate length direction and LETT in width direction decreases with decreasingS content. However, even whenS is decreased to 9×10^-6, the difference of the LETT is still 16℃. When TO varies between 1.8×10^-5 and 5.2×10^-5, the low temperature impact toughness of the steel plates slowly decreases with TO increasing. When TO increases to more than 5.2×10^-5, the low temperature toughness of the steel rapidly decreases with increasing TO content.