A Si(100) crystal implanted with 5x10(16) cm(-2), 140 keV He+ ions was probed using a slow positron beam and the defect distribution along depth was obtained from the relation between S parameter and positron incidence energy. The near surface region of implanted specimen was slightly damaged. Vacancies and small vacancy clusters less than 1 nm in diameter were the dominant defects, while the deeper region around the He+ projected range was heavily damaged and had larger helium microbubbles and microvoids concentration. Study of isothermal annealing at different temperature shows that low temperature anneal can remove most vacancy-type defects efficiently. However, annealing at high temperature enlarges the diameters of microbubbles and microvoids.