This paper investigates the effects of opening existence on the behavior of Post-tensioned flat slabs considering different opening locations and by using the finite element method. The design status, strip moments, Stress distribution around the opening and Long-term deflection are studied. Results indicated that, in most cases, significant decrease of bending moments in strips at which openings are located and this decrease is increased by increasing the opening size. Maximum stresses around the opening’s perimeter and Maximum long-term deflection are increased by increasing the opening size. The existence of opening at column strip near the column has the worst effect on design status, on the other hand the opening at the center of the exterior panel has the least effect on the stresses around the perimeters of opening and the long-term deflection of the slab behavior. For this case, no spans produce excessive tensile stresses for spans up to 10m and opening size up to 0.25 of span length. The existence of opening at mid of interior or exterior span of column strip shows effects between the two previous openings concerning all parameters