Diamond films were deposited in microwave plasma chemical vapor deposition (MPCVD) method on plain silicon substrates with (100) orientation. And the pinhole defects on them were investigated by optical microscopy and scanning electron microscopy (SEM). X-ray masks were fabricated with the films deposited by us. We found the pinhole defects in the film destroyed the gold absorber. The corrosion-resistance tests conducted in 30% KOH solution under 80 oC showed that the diamond films with pinhole defects have lower corrosion-resistance. In addition, the possible mechanism of the formation of pinhole defects in diamond films was discussed. And we deduced that the defects on substrates, competitive growth of multi-phase in diamond films, lattice dislocation between substrates and diamond films could be associated with the defect formation.