In next generation wireless networks, a variety of heterogeneous radio access technologies are expected to be available simultaneously within a single wireless terminal. This paper addresses the challenge of link aggregation in multi-access networks, where a key issue is how to optimally aggregate bandwidth offered by the individual radio link. Effective link throughput model is employed as a common resource measure for heterogeneous links. Based on the model, we present a packet allocation technique to optimize link aggregation. The mapping of packets to radio access links is performed to maximize the overall expected system throughput. Airtime constraint is added for an equal load balance. We have conducted extensive simulations with two scenarios. One is at the co-existence of WiMAX and WiFi, and the other includes only WiFi links. The proposed technique adaptively operates to time-varying link resource, and achieves better the aggregated link performance than WRR with fixed distribution ratio.