Probing attack is a severe threat for the security of hardware cryptographic modules (HCMs). In this paper, we make the first step to evaluate the vulnerability of HCMs against probing attack, wherein we investigate the probing complexity and the key candidate reduction capability for probing attack on every signal in the circuit. We also present approximate solutions for the calculation of the proposed metrics to reduce computational complexity. Experimental results demonstrate that the proposed evaluation metric is both effective and efficient.