Player identification is an outcome of gameplay experiences in virtual worlds and has been shown to affect enjoyment and reduce self-discrepancy. Avatar customization has potential to impact player identification by shaping the relationship between the player and the character. This mixed method study examines the effects of avatar-based customization on players' identification with their characters, and the effects of identification dimensions (i.e., perceived similarity, wishful identification, embodied presence) on their motivation in a massively multiplayer online game, Lord of the Rings Online (LotRO). Participants (N = 66) played LotRO either in customization or in no-customization group for ten hours in four sessions in a lab setting. Data were collected through interviews and surveys. Results showed both time and avatar customization positively impacted player identification with their characters. Player motivation was predicted in different sessions by different identification dimensions, which shows the dynamic and situational impact of identification on motivation.