This paper studies loss performance of optical burst switching (OBS) with assembled burst traffic input by exploring the characteristics of assembled burst traffic. We analyze the smoothing effect of assembly algorithms, which changes the statistical properties of packet flows, and and that assembly algorithms smooth the short range burstiness in packet traffic and thus enhance the loss performance in OBS. Based on the characteristics of such assembled burst traffic, an accurate loss model for OBS is provided, which works much better than traditional loss models in packet switching networks.