We evaluate the vulnerability of a pipelined Advanced Encryption Standard (AES) against Correlation Power Analysis (CPA) Side-Channel Attack (SCA). We identify that the registers in pipelined AES are most vulnerable against CPA SCA and propose a new power model targeting on the switching activities of the registers. The proposed power model is constructed based on the Hamming Distance (HD) between the intermediate values stored in the registers in two consecutive clock cycles. Then, we analyze the vulnerability of pipelined AES under two scenarios, regular pipeline operation and non-pipeline operation where delays are inserted. The simulation results show that under regular pipelined operation, our proposed power model can reveal all the 16 key bytes in less than 4,900 traces, resulting in 4.7× more effective than the conventional power models. Under non-pipelined operation, our proposed power model requires only 590 traces to reveal all the 16 key bytes, which is 5.9× more effective than other power models.