In this paper, a biconical tapered fiber sensor is applied to monitor the temperature of cement paste during curing process. A simple sensing principle of the proposed sensor is presented by analyzing the V-number of leaky rays. An experimental is carried out by tapering the plastic multi-mode step-index fiber to measure the temperature of the cement paste. The numerical result shows that the V-number of leaky ray is increasing with the raised in temperature. The experimental results are in agreement with the results from the theoretical analysis.