A wireless apparatus and an interference determination method thereof are provided. The wireless apparatus determines that the wireless apparatus operates in a first bandwidth mode, and counts a first clear channel assessment (CCA) number associated with a primary channel, a second CCA number associated with a secondary channel, an entire CCA number associated with either the primary channel or the secondary channel, and the false alarm number according to at least one received radio frequency (RF) signal. Afterwards, the wireless apparatus determines that the second CCA number exceeds the summation of the first CCA number and bias number to determine that it is in an interference state, and determines that the interference state belongs to either the first interference type or second interference type according to the radio of the entire CCA number to the FA number.