An analytical expression for instantaneous thermal efficiency (eta(i)) of a greenhouse in terms of design and climatic parameters has been derived by considering the energy balance equations for different components of the greenhouse. The present analysis can be used for testing the suitability of greenhouses of different sizes and shapes for given climatic conditions. Numerical computations for determination of instantaneous thermal efficiency (eta(i)) and greenhouse efficiency factor (F＇) have been carried out for a typical greenhouse. Parametric studies have also been carried out to predict the performance of the system. Experimental validation has also been reported for a climatic condition of Delhi. It is observed that: 1. there is a fair agreement between theoretical and experimental observations, and 2. the instantaneous thermal efficiency (eta(i)) increases with increase of relative humidity (gamma) due to less evaporation as expected.