A transient mathematical model is presented for a box-type solar cooker with a onestep outer reflector hinged at the top of the cooker. It is based on analytical solution of the energy balance equations using Cramer＇s rule for different elements of the cooker. The various heat-transfer coefficients are assumed to be temperature dependent. To validate the model, the temperature distribution obtained by computer simulation is compared with experimental results which were obtained in previous work for a typical summer day (1/7/1992) in Tanta, latitude 30 degrees 47＇N, Egypt. Good agreement between experimental and theoretical results is observed. The boiling and characteristic boiling times of the cooker are decreased by 50 and 30%, respectively, on using the cooker around midday.