Quantitativeness of RHEED intensity as a high speed tool to monitor nucleation and growth of 7×7 DAS domains on Si(111) surface rapidly quenched from high temperature disordered 1×1 phase has been investigated by comparing the time dependences of both RHEED intensity and STM images. The superlattice reflection intensity of RHEED normalized with that of uniform coverage increased gradually as a function of time after the quenching. The increasing rate was higher for higher substrate temperature. A series of STM images revealed the transient aspect of the Si(111)7×7 reconstruction that the plural nuclei were formed at random at the initial stage of growth, and the domain density decreased by coalescence as the growth proceeded. The coverage by 7×7 DAS domains obtained from STM images were always higher than the normalized intensity of superlattice reflection, which shows that the intensity was reduced due to interference between randomly located DAS domains.