‘C’ 0.30, ‘Si’ 1.01, ‘Mn’ 0.88, ‘Cr’ 0.86 is a high strength low alloy (HSLA) steel with < 5 % alloying elements contents and tensile properties in order of 1100 ~1300 MPa in quenched and tempered state. One of the applications of these steels is casing materials in large welded pressure vessels due to high strength and good weldability. ‘C’ 0.30, ‘Si’ 1.01, ‘Mn’ 0.88, ‘Cr’ 0.86 steel sheets (d=4.00 mm) are rolled, welded and stress relived during manufacturing of pressure vessels. The pressure vessel is finally quenched at 880±100C for 90 min and tempered at 480±100C for 50 min in a controlled atmosphere. The specified values of tensile strength, % Elongation and impact toughness after the final heat treatment are ³ 1100 MPa, ³ 10 % and ³ 50 J/cm2. Decarburization affects the mechanical properties of these steels and for 4.00 mm thick sheet decarburization depth of ³ 0.2 mm is permitted after the final heat treatment. In some samples decarburization depth > 0.2 mm was observed, the reason for which is not included in scope of this paper. The current work determines the effect of decarburization depth on the mechanical properties of ‘C’ 0.30, ‘Si’ 1.01, ‘Mn’ 0.88, ‘Cr’ 0.86 HSLA steel sheet (d=4.00 mm) 05 samples of ‘C’ 0.30, ‘Si’ 1.01, ‘Mn’ 0.88, ‘Cr’ 0.86 steel sheet (d=4.00 mm), with decarburization depth varying between 0.02mm to 0.5 mm, have been selected in the current work. The mechanical properties studied are tensile strength, yield strength, % elongation and impact toughness. It has been observed as the decarburization depth increases from 0.02mm to 0.50 mm the tensile strength decreases from 1311 MPa to 1056 MPa and the Impact toughness increases from 77 J/cm2 to 109 J/cm2.