The aerodynamic mechanism of early transition phenomena over an ablating reentry capsule has been analytically examined. A two-equation turbulence model (- model) was coupled with Reynolds averaged Navier-Stokes equations. Low-Reynolds-number effects on the solid wall were taken into account by modifying the Chien's correction. As a result, transition occurred at the lower Reynolds number with higher ablation rate. The predicted transition-point Reynolds number was 3×104 at the surface-mass-injection rate of 100 g/sm2. The principal mechanism of this early transition is thought as follows; the viscosity damping effects are reduced and re-laminarization is prevented in the downstream of the capsule surface, due to the turbulence on the surface and due to the pushing out of near-surface stream-lines from the surface by successive mass injection.