The effect of the heat treatment on mechanical properties of the A356 alloy was verified in course of tensile tests. Keeping of the alloy during 0,5 to 3 hours in temperature 520 - 550 °C, and subsequent ageing in temperature 165 °C during 5 ÷ 8 hours, enables obtainment of maximal tensile strength Rm. In case of the elongation A5, ageing is of the main importance, and should be performed in temperature 325 °C during 2 ÷ 5 hours