The low frequency noise in polycrystalline silicon thin-film transistors (TFTs) has its origin in both sources: (i) the Si/SiO2 interface; and (ii) the grain boundaries present within the transistor channel. A model is developed which takes into account the fluctuations of carriers in both sources of noise. Noise is measured in two types of TFTs. In one type, the poly-Si layer has a low in-grain defect density (5 X 10(8) cm(-2)) and an average grain size of the order of 100 nm. In the other type of device the average grain size is of the order of 2500 nm while the in-grain defects density varies from 5 X 10(10) cm(-2) to 5 X 10(12) cm(-2). The noise analysis shows that for the first type, the Si/SiO2 dominates while for the second type of device, the two sources of noise are present. The model also gives an estimation of the electronic parameters of the device.