There are two passive microwave sensors onboard the Water Cycle Observation Mission (WCOM), which includes a synthetic aperture radiometer operating at L-S-C bands and a scanning microwave radiometer operating from C- to W-bands. It provides a unique opportunity to disaggregate L-band brightness temperature (soil moisture) with S-band C-bands radiometer. In this study, passive-only downscaling methodologies are developed and evaluated. Based on the radiative transfer modeling, it was found that the TBs (brightness temperature) between the L- and S-bands exhibit a linear relationship, and there is an exponential relationship between L- and C-bands. The downscaling method with L-S bands with the same incident angle was first evaluated. The RMSE are 3.19 K and 1.98 K for H and V polarization respectively. The downscaling method with L-C bands is developed with different incident angles. The RMSE are 2.97 K and 2.68 K for H and V polarization respectively. These results showed that high-resolution L-band brightness temperature and soil moisture products could be generated from the future WCOM passive-only observations.