Abstract:As far as daily insolation on slope is concerned, a critical slope angle may exist where the daily insolation on slope is equal to that on the horizontal surface for the given latitude, slope angle and period. Critical slope angle exists not only on the slope toward equator but on the slope entirely shaded from the sun. Generally, there is only one critical slope gradient in the low and middle latitude slope. From the horizontal surface to the critical slope gradient, the daily insolation on slope is much more than that on the horizontal surface . Specially, when the latitude is 0-45° at equinoxes, the critical slope gradient of the slope toward equator is equal to two times of its latitude which the slope belongs to, while two or even three of critical slope angles within the polar circles are possible.