InAs is desirable for a single nucleus in every selected growth area in micro-channel selective-area MOVPE on Si(111) substrates. In order to obtain the uniformity in the crystal shape of InGaAs micro-discs, we have devised a multi-step growth which starts from the InAs nucleation step. In this study, we have investigated the dependence of InAs nucleation on the partial pressure of an In source (PTMIn) with in situ monitoring to cover the Si surfaces by the flat InAs crystals. Since the initial nucleation of InAs is governed by the supersaturation of an In precursor on the Si surface, higher P TMIN is necessary. On the other hand, smaller PTMIn is suitable for burying the growth area by lateral growth. Therefore, we should modulate PTMIn during the InAs growth: PTMIn should be decreased as soon as the initial nucleation occurs.