In this work a congestion level based optical burst assembly technique is proposed to be used in optical burst switching (OBS) networks. The effect of adaptive burstification duration (time threshold) and adaptive size threshold on the performance of OBS channel scheduling is analyzed under various traffic types. The proposed burst assembly technique varies (adapts) the time and size threshold values of the virtual queues at optical nodes considering the link congestion levels. Loss rates on the optical links are used to represent congestion levels and to arrange the time threshold and the size threshold values on the nodes which am incident with them. Three alternative size-time threshold values are employed based on the congestion level. Under heavy traffic, smaller threshold values are used. As the traffic becomes lighter, the threshold values are increased to allow longer burst generation and better channel utilization. Moreover, in order to make the implementation of optical nodes simpler, only the bursts of size mu(min), SThshort, SThmedium, or SThlong are generated. In the OBS network, a well-known and practical OBS technique, namely Horizon, is employed. The results are evaluated in terms of byte loss rate and compared with those obtained using a static (nonadaptive) burst assembly scheme. It is observed that the proposed technique dramatically decreases the overall byte loss rate while keeping the end-to-end delay in a feasible range.