Abstract: To investigate the partial volume effect (PVE) and its influence factors for sphere lesion positron emission tomography (PET) images and provide the basis for PVE correction, the computer simulation experiments for PET imaging were performed. According to the theory of PVE formation, PET images of the sphere lesions with diameter of 2.30 mm were simulated with computer program in the ratio of background to lesion of 0-0.7 under system resolution of 3.15 mm. The maximum and the average pixel values and the width of the lesion images were calculated. The results show that the maximum and average pixel values increase with the lesion diameter in S shape under every system resolution and background. The maximum and average pixel values decrease in inverse S shape with the increasing of the system resolution under every lesion diameter and background. The maximum and average pixel values linearly increase with the increasing of background under every lesion diameter and system resolution. The width of the lesion image is independent of the background under every lesion diameter and system resolution. The FWHM of the lesion image is close to the lesion diameter for the lesion which size is larger than system resolution and the difference increases with the system resolution value, it doesn’t change with the lesions which size is smaller than system resolution and it is equal to the system resolution.