To study the effect of the shield thickness of the surrounding on the total and photo peak relativity efficiency for the NaI(Tl),two NaI(Tl) detectors (3'' × 3'') and (1.5" × 1.5") crystals are used and the radioactive source is Cs-137 which emits (0.662) MeV photons at distance of(25)cm from the detector and (146.8µci) activity.The Surrounded materials represented by cylinders of (35) cm. long, and different thicknesses (2.5, 4.4, 5.6and8.9) mm. and different diameters (11, 16 and 21.5) cm. for the steel (Iron) cylinder.For (1.5" × 1.5") and (3'' × 3'') detectors without cylinder, the total and Photo peck Relativity efficiency are (100%) relative to it selves.While, increasing of the thickness of the cylinder, the total and Photo Peak Relativity efficiency decrease for the two-size detector