Sulphate attack is a significant problem which locally affects concrete and other constructional materials. The aim of this work is to study the effect of internal sulphate which is present in concrete raw materials. Gypsum was used as a source of internal sulphate. Specimens were prepared for mechanical tests in formed of (150mm cubes) for compressive test and (100*100*400 mm prisms) for flexure test with selective mix proportions of 1:2:4 by weight and water/cement ratio of 0.5 by weight. Results have shown that the compressive strength of concrete at age of 60 days decreases by 46%, 52%, 65% when added sulphate is in 3%, 5%, and 7% by weight of sand to concrete mixes, respectively. Also results have shown that when the percentage of cement content increased by 3% , 5% , 7% in concrete mixes ,which contain sulphates in 3% , 5% , 7% by weight of sand ,the compressive strength of concrete at 28 and 60 days has been increased and has the same compressive strength of concrete without sulphates. Moreover, flexural strength decreased by 10%, 25% and 38% when added sulphate is in 3% , 5%, 7% by weight of sand to concrete mixes and at age of 60 days, respectively. This decrement in flexural strength has been tempered by increasing the cement percentage by (3, 5, 7) % of cement weight to become equal to the value of standard specimens.