Slow positron beam technique has been employed to study the defects of graphite, nanophase C, as well as the structural changes in un-doped, B-doped and S-doped diamond films with annealing temperatures. The results show that the concentration of defects in nanophase C is higher than that in graphite. The vacancy concentration in the S-doped diamond film is higher than that in un-doped one. The addition of small amount of B atoms leads to the decrease of the vacancy concentration in the film. The vacancy concentration in un-doped diamond film would decrease after annealing at temperature below 600°C, while vacancy concentration will increase after annealing at temperatures above 900°C. The vacancy concentration in the 40 Ω cm B-doped diamond film decreases at annealing temperatures higher than 200°C.