In this paper thermal performance test experiments for first figure of merit (without load) and second figure of merit (with load) of a box type solar cooker were conducted as per Bureau of Indian Standards. The values of second figure of merit (F-2) were determined for different loads of water and the results show that F-2 depends on the quantity of water loaded in a solar cooker. Therefore, it is recommended that the performance test method should specify the amount of water which is to be taken.