In order to perform reactor core analysis with high accuracy, appropriate resonance treatment method plays a very important role within the whole calculation procedures. On the other hand, the fast reactor designs are the best choices to solve future energy problem. Therefore, the in-house code STREAM, which is initially developed for light water reactor calculation, is decided to upgrade for fast reactor analysis. Current work implements the Improved Tone’s Method into the STREAM code to perform 2-D pin-cell/assembly calculation. Several verification tests show the accuracy of new STREAM code, with less than 50 pcm difference on keff value.