Abstract: In order to explain theoretically the difference of observed cloud intensities and structures by TRMM PR and ground-based radar, the effects of wavelengths, orientations of radar waves and radar beam widthonthe observations of reflectivity were simulated. The results show that the errordue to the different wavelength and orientation of radar wave is within 2.0 dBz, TRMM PR can observe larger reflectivity than ground-based radar in echo center. TRMM PR smoothes the cloud structure, overestimates and underestimates 3-5 dBz in strong and week echo areas, respectively. Beam width and long distancefrom TRMM PR to target cause TRMM PR to observe large echo area and area integrated rainfall amount and weak averaged echo intensity. The theoretical results above can only explain part of observed facts, which means that the comparison of observation result between TRMM PR and ground based radar is complicated, the attenuation of radar wave within precipitation area is the main factor to affect the observed result.