A technique of three-dimensional (3D) CT image simulation based on the point spread function (PSF) was developed. We simulated CT images from the object functions of ideal spheres, assuming solitary pulmonary nodules, and calculated the sphere volumes with a simple threshold technique in which the relationship between the threshold and the volume was analyzed. We determined the threshold (defined as T') so that the sphere volume obtained in simulated image was equal to its object function (true volume). It was quantitatively shown that the T' changed with the object sphere diameter, the image reconstruction kernel, slice thickness and slice location while maintaining a constant density in the object. Our simulation technique and present results will be effective for basic research on 3D nodule analysis.