PURPOSE: To determine the effect of angle variation, relative to scan plan and optimal window setting, on thesize of three dimensional spiral CT images of a tracheobronchial tree using a phantom. MATERIALS AND METHODS:Spiral CT[collimation(mm)/table speed(mm/sec): 3/3; reconstruction interval:1.5 mm] was performed on atracheobronchial phantom consisting of a box filled with saline solution containing air-filled 5cc syringesoriented at varying angles relative to the scan plan. The diameter of three dimensional images was measured ateach window(minimal threshold value; -1000 HU; maximaml threshold value: from -300 HU to -500 HU; 50 HU interval). RESULTS: The inner diamenter of syringes used for tracheobronchial phantom was 12.55 +/-3.19mm. At all windows,as the angle became more perpendicular, the diameter of three-dimensional images increased, and at all angles, asmaxinal HU increased, measured diameter also increased(p<0.0001). In particular, at a maximal value of -500HU,measured values were closest to control values at all angles. CONCLUSION: Diameter can be measured close tocontrol value from three dimensional spiral CT images at maximal HU of -500 window, regardless of angle.