By taking the vectorial property of time average energy flow density and the accurate formula of the light intensity into accout, the first and second intensity moments of the nonparaxial scalar beams are defined. Based on the static scalar wave equation, the propagation law of the second intensity moments of monochromatical scalar beams in free space are obtained. A new beam qualty factor for nonparaxial scalar beams is introduced. Moreover, some important aspects of the intensity moments theory are discused.