Method and equipment to control operating temperature of an air compressor. A compressor element compresses air and oil and supplies it to an oil separator. In the separator, the air and oil are separated. Oil is led to a circulating pipe and returned to the compressor element. When necessary, at least some of the oil flowing in the oil circulating pipe is supplied to cooling, which is used for controlling operating temperature of the compressor such that it is as low as possible, but nevertheless so high that no condensation point is reached. The amount of oil to be supplied to cooling is controlled by a thermostatic valve based on a change in dimension of a controlling element such that dimensions of the controlling element are changed by an external command.