1/f noise in n-channel and p-channel metal_oxide_semiconductor field effect tran sistors(MOSFETs) with different channel areas, are studied in theory and experim ent. Experimental results demonstrate that the 1/f noise magnitude in n-channel MOSFETs is ten times than that in p_channel MOSFETs, but both magnitudes are al l in inverse proportion to the power of effective gate voltage and its active ar ea, while they are in direct proportion to the power of drain voltage. Based on these experimental results, the mechanisms are discussed that the 1/f noise in M OSFETs is attributed to the random trapping/detrapping processes between channel and oxide traps near the SiO2_Si interface about several nanometers, which ca use fluctuations in both the number and the mobility of channel carriers. Based on these mechanisms, a unified 1/f noise model for MOSFETs is developed, includi ng tunneling and thermal activation through which traps communicate carriers wit h channel. Experimental results agree well with the developed model.