Phase truncation is a supplemental technique to help decrease the delay at signalized intersections in unusual flow restriction, by re-allocating the unusable green time to other phases. Previous studies have shown how to implement phase truncation in NEMA controllers with multiple rings and reported it to work well. Nevertheless, it is unclear whether phase truncation will always be beneficial. With microscopic traffic simulation software package, VISSIM, this paper simulated a wide range of traffic scenarios at a typical four-leg signalized intersection equipped with phase truncation. With discriminant analysis method, it is believed that decision of installing phase truncation should be carefully taken according to the local traffic patterns.