Street canyons become a common feature in many countries due to a rapid increase in the population densities in many cities around the world. Noise control at sources is one of the most effective methods from a physical point of view. However, it is difficult, if not impossible, to implement this idea in practice. A possible solution is to understand the acoustic environment of a street canyon before noise control measures are implemented in this complex urban environment. The reverberation time is often used as an index to assess the acoustic environment of the street canyon but there are relatively few studies dedicated to the development of a simple but accurate formula for its prediction. Based on an energy approach, an integral equation and then a simple approximate formula have been developed to predict the reverberation times for a street canyon in the present study. The predicted results of the proposed formulae are compared well with experimental and theoretical results obtained from previous studies.