Low-level water clouds are recognized as an important cloud type in a climate system since the shortwave cloud radiative forcing of low-level cloud is large and the low-level clouds cover huge areas off the west coast of the continents.Therefore, diurnal variation of optically thin water cloud off the west coast of Namibia was investigated using the multi-channel data of Meteosat-8. Water cloud can be identified using the 8.7 and 11 micron data. The brightness temperature difference between the split window (BTD) of water cloud was compared with the optical thickness derived from so-called solar reflection method which uses 0.6, 3.7 and 11 micron. Good relationship was found between the two parameters. The BTD decreases with the increase of optical thickness. Therefore, the diurnal variation of optical thickness of water cloud was studied using the BTD from the hourly data of Meteosat-8. The BTD showed minimum value around early morning and showed maximum value around late afternoon. The local time of minimum BTD is roughly corresponds to that of the maximum cloud cover. While solar reflection method showed maximum optical thickness around noon. Further, we studied the feasibility to derive both optical thickness and effective radius using the split window data. The theoretical computation of TBB and BTD for optically thin water cloud changing the optical thickness and effective radius becomes arch shape in TBB-BTD diagram. The arch shape differs depending on effective particle size as seen for ice clouds. Generally the cloud top over the west coast of continents is capped by the strong inversion layer. Therefore, it is easy to determine the cloud top temperature from the optically thick cloud pixel within the analysis area. Using the look-up table computed in advance for the TBB and BTD changing the optical thickness and effective radius, we can derive the optical thickness and effective radius from the observed TBB and BTD. The effective radius tends to become smaller from the early morning to late afternoon.