Pistons used in automobile engines are made of aluminum alloy; and endurance of high pressure and high temperature is required.Recently,high strength pistons are needed to cope with the increase of pressure and temperature in the engine cylinder.A high strength piston is possible when the piston casting has little or no casting defects,such as micro-pores and cracks.Generally the defects can be evaluated by non-destructive testing (NDT),and the most efficient ways for evaluation are using ultrasonics and computer tomography.In the present study,two NDT methods were compared and evaluated to investigate the defects in the aluminum piston castings.Artificial defects were machined by using very small drill bits with diameters 1,0.5,0.3,0.2 and 0.1 mm.The defects were investigated by using an ultrasonic tester and a computer tomography system.Defects smaller than 0.3 mm may not be found by using the ultrasonic test,but the defects may be found by using the computer tomography system.The investigation conditions for detecting small defects and the scanning time for in-line defect analysis are discussed in the present study.