Two new parameters helpful in determining the center position of tropical cyclones (TC) using the TRMM (Tropical Rainfall Measurement Mission) Microwave Imager (TMI) data have been devised. The new parameters utilize the differential responses of the center and surrounding eye-wall of a tropical cyclone to the upwelling microwave radiation in two different channels in terms of intensity and polarization. One uses the intensity difference between 37 and 85 GHz and another uses different characteristics of polarization at 37 GHz. Case studies show advantages of the derived parameters over the single channel microwave or optical imagery, especially when high clouds cover the center. When the TC center determined with the two parameters is compared with the best track position reported by JTWC (Joint Typhoon Warning Center) and RSMC (Regional Specialized Meteorological Center) Tokyo, the mean differences are about 39 km and 41 km, respectively, for 91cases detected in the east Asia region by TMI during 1999 and 2000. For the same cases, the mean difference of the center position between RSMC and JTWC is about 23 km.