Electromagnetic (EM) field measurement uses antenna in receiving mode as EM field sensor (EMFS). FDTD technique is applied to analyze the horn antenna in this mode. The performances of horn antenna as EMFS are predicted in terms of antenna factor (AF), and the results are compared with measurements. New empirical formula for quick calculation of antenna factor of pyramidal horn is also presented. Also, the gain of horn antennas are computed in receiving mode using FDTD technique, and the results are compared with published measured result and far-field gain formula.