The velocity field in a wide rectangular channel (AR = 4) with staggered circular pin fin array was measured using Particle Image Velocimetry (PIV). The cylinder pin fin has a diameter of 8 mm and height to diameter ratio (H/D) of 3. The streamwise pitch (X/D) and spanwise pitch (S/D) of pin fins are 4.5 and 2 respectively. The experiment was conducted at two Reynolds number, 10000 and 20000, based on the hydraulic diameter and bulk velocity of the channel. The mean velocity, first-order statistics of velocity fluctuation and turbulent kinetic energy components in the measured plane are given. Results show, the distribution of the cross-stream velocity fluctuation (Vrms), other than second-order statistics, has a similar pattern as that of the local Nusselt number on the end wall between pin fin rows while the flow is in developing. The Nusselt number was achieved through temperature measurement using thermochromic liquid crystal in the same geometry setup. Therefore, compared to second-order statistics, the cross-stream velocity fluctuation might be a simple but crucial flow quantity for heat transfer enhancement using pin fins in the developing region. The shedding of large vortices from circular pin fins is believed to be the primary flow structure inducing the cross-stream velocity fluctuation. Study on pin fins with various geometries is required before further conclusion drawn.