This paper presents the field uniformity characteristics in a triangular prism reverberation chamber and a rectangular reverberation chamber that can be substituted for an open area test site or an anechoic chamber to measure electromagnetic interference. To improve size problems of a stirrer that is an essential unit to generate a uniform field in the reverberation chamber, we suggest a diffuser of Quadratic Residue Sequence method. To validate the substitution of a diffuser for a stirrer, a diffuser is designed for 1-3 GHz, and five types of equilateral triangular prism reverberation chambers and three types of rectangular reverberation chambers are modeled. Afterwards, the field distributions in these eight reverberation chambers are simulated and tested. Using XFDTD 6. 2 of finite difference time domain method, field deviations of each structure are simulated and compared to each other. An evaluation of field uniformity is done by cumulative probability distribution as specified in the IEC 61000-4-21. The result shows that the field uniformity in the chamber is within ±6 dB tolerance and also within ±3 dB standard deviation, which means a diffuser can satisfy the requirement of international standards.