This paper introduces an approach of novel principle to measure the directional emissivity of material surfaces direct. The value of the infrared directional emissivity is obtained by comparing the surface spectral emissivity of testing samples to a blackbody reference. Weak signals are measured with pyroelectric detector by optical modulation and phase-locked amplified technologies. Our experiment is implemented with stainless steel in circumstance of 20 - 300°C, the testing sample is 8µm- 14μm. Compare to the reference and the calibration value without a cooling process, combined uncertainty is less than 0.04, and the result is satisfied.