Abstract: To obtain definite exposure value under the precondition of fixed mA value and for certain object radiographic arrangement and X-ray unit, two variables of the radiation energy and exposure time are inter-related. The products of the square of tube voltage and exposure time are correlative as two variables varied before and after. The thinking of making certain of correction factor Kt and its value were presented. The formula of V21·t1=Kt·V22·t2 for some validation and points out its possible practical applications.