Conventional optical burst switching (OBS) technique adopts purely statistical multiplex mechanism so that the bursts collide with each other very easily. To address this problem, a novel proactive contention avoidance scheme is proposed, which assigns dedicated wavelengths to each ingress node, then st-numbering algorithm is used to construct the traffic load balanced spanning trees. In this way, contention can be eliminated at ingress nodes, and the amount of bursts that could be accommodated by ingress nodes will be maximized. Further, those unused wavelengths left by traffic load balanced spanning tree are also organized as partial trees to carry bursts, thus the link utilization can be improved effectively. Simulation result shows that our scheme can improve the burst loss performance significantly without the wavelength converters or optical buffers comparing to other popular routing and wavelength assignment (RWA) algorithms.