A steel sheet excellent in mechanical strength, workability and thermal stability and suited for use as a raw material in such fields of manufacturing automobiles, household electric appliances and machine structures and of constructing buildings, and a manufacturing method thereof are provided. The steel sheet is a hot-rolled steel sheet of carbon steel or low-alloy steel, the main phase of which is ferrite, and is characterized in that the average ferrite crystal grain diameter D (.mu.m) at the depth of 1/4 of the sheet thickness from the steel sheet surface satisfies the relations respectively defined by the formulas (1) and (2) given below and the increase rate X (.mu.m/min) in average ferrite crystal grain diameter at 700.degree. C. at the depth of 1/4 of the sheet thickness from the steel sheet surface and said average crystal diameter D (.mu.m) satisfy the relation defined by the formula (3) given below: 1.2.ltoreq.D.ltoreq.7 formula (1) D.ltoreq.2.7+5000/(5+350C+40Mn).sup.2 formula (2) DX.ltoreq.0.1 formula (3) wherein C and Mn represent the contents (in % by mass) of the respective elements in the steel.