A mathematical model is devised to study the thermal behavior of a greenhouse while heating with a ground air collector (GAC). A computer program based on MatLab software has been used to predict the plant and room temperatures as a function of various design parameters of the ground air collector. Extensive experiments have been conducted during December 2000 to March 2001 for an even span greenhouse of effective floor area of 24 m(2) with a GAC and having a brick north wall. The model was validated experimentally in the climate of Delhi for the winter season. A parametric study involves the area of the GAC, mass flow rate and heat capacity. The predicted plant and room temperatures show fair agreement with the experimental values.