MP3 is the most popular audio format nowadays in our daily life, e.g., music on the Internet are often of this format. However, low bit rate MP3s are sometimes transcoded to high bit rate, resulting in fake-quality MP3, to seek commercial benefit. This paper presents a method to defeat this kind of fake-quality MP3. Our experimental study and theoretical analysis have demonstrated that there are fewer MDCT coefficients of small values in fake-quality MP3 than in normal MP3. We use the numbers of small-value MDCT coefficients as features to discriminate fake-quality MP3 from normal MP3. Experimental results show the effectiveness of the proposed method. To our best knowledge, this work is the first one to find out fake-quality MP3.