The heavy rainfall occurred at Hong Kong and vicinity on Aug. 23,1999 was caused by typhoon ″Sam″. The rain rate and reflectivity distribution at 1km observed by TRMM/PR and HK radar are compared. Generally,they are consistent. Two methods, i.e. average and best match, are used in comparison. The correlation coefficients obtained from best match method are higher than average method. When the distance from HK radar is less than 100km the coefficient is higher. The difference of reflectivity from PR and HK radar has increased with the distance increasing from radar site. When the reflectivity from PR is less than 20dB the value observed by HK radar is larger. Otherwise, the reflectivity from PR is larger. The difference of reflectivity between the two radars increases when the reflectivity from PR becomes larger.