Integration of InGaAs/InAs nanowire on silicon (Si) substrate has been attracting huge attention for opto- and micro-electronics applications. In this work we report selective area epitaxy (SAE) of InGaAs/InAs heterostructure (HS) on Si (111) using metal organic chemical vapor deposition (MOCVD). High quality InAs wurtzite (WZ) segment is grown axially on InGaAs NW when the arsine (AsH3) supply is low. The growth phenomenon behind the axial InAs HS on InGaAs is attributed due to surface segregation and sidewall diffusion of indium (In) adatoms on the surface of the InGaAs nanowire at In-rich/arsenic (As)-limited region. For high AsH3 flow, As coverage impedes InAs segment formation.