A two-group albedo theory is developed which seems to be valid for the calculation of temperature coefficients of nuclear reactors characterized by the PSR. Measurements of over-all coefficient for the PSR are in qualitative agreement with results calculated by the theory. Analysis under the present theory singles out the temperature variation of the ratio of the age to the thermal neutron diffusion length of the reflector as the primary contributor to a low temperature positive coefficient effect. The advantage of representing the criticality factor, k, by the two-group albedo theory is well illustrated by the endeavor of calculating the temperature coefficient. Under normal two-group multiregion treatment the criticality factor never explicitly appears and therefore it is impossible to obtain an explicit form of the variation of k with system parameters. The dissection of the nonleakage probability in the present theory is not unique, but it does lead to easy physical interpretation.