In order to examine the role of structural vacancies in the stress-induced phase transitions of B2-type FeRh alloys, Fe-40,45 and 50at%Rh specimens were deformed at room temperature with a high-speed compression machine and were studied by X-ray diffractometry (XRD) and positron annihilation measurements. It was found from the positron lifetime results that vacancies or vacancy clusters were introduced into die alloy by deformation. The longer lifetime (tau (2)) components were changed with the deformation momenta and Rh concentrations. In the case of Fe-50atO%Rh, they were 188 ps and 254 ps after deformation with small or large momenta, respectively Taking the X-ray results into consideration, it is concluded that an atom movement mechanism forming large vacancy clusters is associated with the B2-Al transition. The short lifetime (ri) of die alloy is accounted for by bulk annihilation in the transformed phases and a high density of dislocations.