Thirteen simply supported steel samples have been tested to explaine the effects of strengthening of steel beams by using a external pre-stressing strand. The samples have the same cross sectional dimensions and overall length. One steel beam without strengthening taken as a reference while the other tweleve of them had been strengthening by two external strands at various eccentricity locations and jacking stresses. The strengthening by external prestressing strands are sub-divided into two series according to jacking stress. Each series consists of six steel sample divided according to eccentricity location of prestressing strand. Durng tests, it was found that the Load_deflection response for the strenegened samples are stiffer as compared with reference, the increasing percentage in ultimate load capacity were increased to 0.347 %, 2.758%, 3.921%, 8.898 %, 9.326% and 10.256% for beams under jacking stress of 1120 MPa, while, the increasing percentage in ultimate load capacity were increased to 0.17 %, 26%, 33%, 48.5 %, 13.7% and 69.56% for beams under jacking stress of 815 MPa. On the other hand the maximum percentages of deflection were decrease to 4.88 %, 2.44%, 20.62%, 15.0% and 9.7% when the jacking stress increase from 815 to 1120 MPa and the ratio of the quarter to mid span deflection (δ quarter / δ mid) is about 0.528 and 0.497 when jacking stress is 1120 and 815 MPa respectively. So the increasing in jacking stresses from 815 MPa to 1120 MPa will be not preferable to be use because it has a little increasing percentage in stiffening and behaviors as compare with other tested beams at the same condition.