MPEG audio coding under the name MP3 has become one of the most popular standards for digital audio broadcasting and videos. High compression ratios offered by MP3 codecs in various stand alone players and hand held devices over the last few years has increased its popularity immensely. Internet users, music lovers who would like to download highly compressed digital audio files at near CD quality are the most benefited. Psychoacoustic model, Modified Discrete Cosine Transform (MDCT) and Huffman coding play a vital role in achieving such magnificent compression ratios. In this thesis, a thorough knowledge of MP3 decoder is obtained by going through the ISO standard and then some of the decoder blocks have been implemented for deeper understanding.