A noise estimator device of a video decoder includes a reception end, a low pass filter coupled to the reception end, a delay unit coupled to the reception end, a minimum level estimation unit coupled to the low pass filter for estimating a minimum of a specific number of low-pass results, a difference level estimation unit coupled to the low pass filter, the delay unit, and the minimum level estimation unit for determining a synchronization signal according to a plurality of low-pass results and the minimum of the specific number of the low-pass results, and for estimating a noise level according to the synchronization signal and signals outputted from the delay unit, and an output end coupled to the difference level estimation unit.