Development of cumulus is studied by numerically integrating the equation of motion, equations of conservation for water vapor mixing ratio, and the thermodynamic energy equation. We use the terrain-following coordinate system called z^*-coordinate system, in which we can easily treat any calculation domain with terrain configuration such as mountains. The model domain of calculation is restricted vertically to 4.8km and horizontally to 100 km, has a bell-type mountain in the centeral part. Four cases are considered, one in a neutral environment, second in a slightly stable environment, third in a environment decreasing water content with low value of initial water vapor mixing ratio, the fourth in a case with higher vapor gradient. The more the atmosphere is unstable, the more cumulus develops easily and the more water vapors is abundant, the more cumulus develops easily too. More detailed cloud microphysics parameterizations and wet deposition must be considered to use in air pollutants prediction model.