Downwelling longwave (LWd) radiation at sea surface is one of the critical components of the heat budget at the surface. It affects sea surface temperature and the heat transport over oceans. Currently, the estimates of the LW radiation from space, such as those in the standard CERES products and ISCCP, are mainly based on the visible (VIS) and infrared (IR) satellite retrievals of optical depth, particle size and cloud height, along with some atmospheric profile and sea surface information. The advantage of this approach is that the estimated LWd fluxes are solely based on physical calculations, and easy to diagnose. The disadvantage is under some conditions, such as cirrus over low clouds or nighttime satellite overpasses, some important cloud properties affecting LWd radiation cannot be estimated using VIS/IR data. This study combines VIS/IR data with satellite microwave measurements over oceans to improve the cloud retrievals from space, especially for multilayered cloud systems and nighttime observations. The data are from TRMM VIRS and TMI instruments during early 1998 (CERES TRMM period), and are for the three CERES validation ocean sites. TMI measurements at 37 and 85 GHz are used to estimate cloud liquid water path (LWP) and cloud water temperature (Tw) for both day and night time views. In this primary study, only clear sky and overcast cases are considered. The satellite averaged LWd fluxes within 100km of the ocean sites are compared with the half hour means of the site in-situ measurements. Both calculated LWd values with and without TMI retrievals agree well with in-situ observations. The bias errors (3.1 w/m^2) with TMI LWP and Tw data are about half of the bias errors (6.7 w/m^2) without TMI data. The rms errors (17.9 w/m^2) using combined VIS/IR/MW data are about 1 w/m^2 smaller than those using VIS/IR only data. The correlation coefficient (~0.6) between the satellite estimated LWd fluxes from combined data and the in situ measurements is also increased. This study shows that the estimations of LWd radiation over oceans are improved with combined VIS/IR/MW data, especially for multilayered clouds with low level water clouds overlapped by higher level cirrus. During nighttime, the microwave estimates of LWP and Tw provide significant information about low level water clouds and increase the accuracy of the estimated downwelling LW radiation.