The most common noises in ECG (electrocardiogram) signal processing are BW (baseline wandering) and the 50 or 60 Hz PLI (power line interferences). In order to remove these two major source of noises, we have used the recent powerful DWT (discrete wavelet transform) signal processing in ECG signals which are obtained from MIT-BIH Arrhythmia Database. The results indicate that DWT is a good method for filtering noises without changing the morphology of ECG, and can be applied to all types of ECG signals, whether normal or presenting arrhythmias.