Dispersion of pollutants in a street canyon is poorly understood mainly because the turbulence inside the canyon is in general unsteady and intermittent. This study attempts to employ large eddy simulation (LES) to gain an insight into the complicated wind structure of the flow. Although there are some previous simulations of street canyon flow with k-e models, the only reported LES of such a problem is two-dimensional. Regional Atmospheric Modelling System (RAMS) is configured for a street canyon with an aspect ratio of width to height of 1/1. Due to the specification of cyclic boundary conditions for three velocity components along x- (cross canyon axis) and y- (along canyon axis) directions, such a configured geometry represents an infinite number of canyons which are infinitely long and parallel to each other. The modelling results are validated against recent wind tunnel experiments. Both mean wind and turbulent kinetic energy (TKE) derived from LES agree well with the observations. A layer of large TKE values is found in the upper part of the canyon near the downstream wall, indicating the active turbulence there. The study shows that LES is a promising novel tool of reproducing the characteristics of turbulence in an idealised street canyon. The results will be used as the input into a dispersion model to study the dispersion of particulate matters inside and above street canyons.