In the diagnosis of prostate disease, urologists examine the presence of fibrosis or tumors in the prostate tissue of patients by magnetic resonance imaging (MRI) and transrectal ultrasonography (TRUS). In general, urologists will imagine the location of fibrosis or tumors in 3D space. To resolve this a fusion algorithm for three-dimensional (3D) prostate magnetic resonance imaging (MRI) and 2D TRUS was developed. Fiducial registration error was employed in this study to evaluate the registration error of the upper half contour. Experiments showed that the proposed method achieved good fusion results. The system can further guide urologists in the performance of TRUS for tissue biopsy or high intensity focused ultrasound (HIFU).