In LTE-Advanced (LTE-A), system enhancements have been studied in the 3GPP (3rd Generation Partnership Project) for standardizing integrated wireless communications using multiple radio access technologies (RATs) toward 5th generation (5G) mobile communications. One of study topics is LTE-WLAN (Wireless LAN) aggregation, aiming at enabling simultaneous usage of LTE radio access and WLAN radio access. An important technical challenge is the existing layer 2 structure. Specifically, the layer 2 structure for LTE-WLAN aggregation should be designed so that the Quality of Service (QoS) level of Evolved Packet System (EPS) bearers is kept even when each EPS bearer is sent over the WLAN radio. Requirements should include: (1) the layer 2 structure must be backward compatible to existing LTE-A specifications, and (2) the layer 2 structure must not have any impacts to existing WLAN specifications. In order to meet these requirements, in this paper, three layer 2 structures are proposed. The characteristics of proposed layer 2 structures are clarified, the pros and cons are discussed, and then one of proposed layer 2 structures is selected as a desirable structure to be standardized in 3GPP for LTE-WLAN aggregation.