Software transactional memory (STM) is an emerging concurrency control mechanism for shared memory accesses. Early abort is one of the important techniques to improve the execution speed of STMs and has been explored intensively via experimental studies. This paper presents a theoretical analysis characterizing the properties of early abort and its impact on the performance of lock-based STMs. Queuing theory is adopted to model the behaviors of transactional execution. Analytical results are obtained for STMs with and without early abort. The analysis is validated through extensive experiments. Our results reveal that although early abort helps improve the performance of lock-based STMs especially when the contention level is low, the gain is often marginal. We expect our theoretical results to provide useful guidance towards the design and selection of appropriate lock-based STM schemes.