A mathematical model MICroclimate of GREENhouse (MICGREEN), consisting of set of algebraic equations, was developed. The equations were written for four components of the greenhouse viz. cover, inside air, canopy surface and bare soil surface. It was assumed that the greenhouse air is well mixed, thermal properties of materials of construction do not change with time and solar radiations pass through cover without absorption. The values of dimensions and material properties of the greenhouse constructed at the Research Farm of Department of Soil and Water Engineering, Punjab Agricultural University, Ludhiana were put in these equations. The inputs to the model are ambient air temperature, solar radiations on normal surface, solar radiations on earth's surface, temperature of the soil under canopy and temperature of the soil at a depth of 6cm. A computer program was written in C++ language. The equations were solved using Gauss–Seidal Iteration method. The outputs of the model are greenhouse cover temperature, inside air temperature, canopy temperature and bare soil temperature. The relative humidity of the inside air is predicted from the predicted inside air temperature with the help of psychrometric chart. To validate this model, experiments were conducted on greenhouse to obtain data during winter as tomato crop was being grown. The results of computer model were compared with the experimental results and agreement was found between the measured and predicted values.