MOSFET's with variable channel lengths have been fabricated in both mono- and fine-grained polycrystalline silicon. We present a new method based upon a simple CV technique, to measure the effective channel length and gate oxide thickness. The channel-length reduction of the poly-Si MOSFET's was about 7.8 µm from which an effective lateral diffusion coefficient at 1000°C of phosphorus of 5 × 10 -13 cm 2 /s was calculated. The electron mobility was in the range of 10-20 cm 2 /V.s and the threshold voltage was about 17 V. The MOSFET's in mono-Si have been used as a reference. The results of measurements on these devices are in agreement with literature.