Emissivity is an essential property in quantitative thermography. A measuring method using the angle dependency of the polarized emissivity is proposed. In this method, the emissivity is obtained by measuring the temperature distribution around the Brewster angle with an infrared thermographic instrument with an infrared polarizer attachment. First, the specimen is heated uniformly then the distribution of the apparent temperature around the Brewster angle is measured by the infrared thermographic instrument with the infrared polarizer attachment. The infrared thermographic instrument can not measure the temperature precisely, but it can obtain the temperature distribution accurately. Next, by making the approximate curve for the temperature distribution, the position of the highest temperature can be identified and then the Brewster angle determined. Finally, using the value of the Brewster angle, the emissivity can be calculated. This proposed method is applied to two insulating materials and obtained good results in comparison with the emissivity obtained by FT-IR. In addition, another technique is proposed that uses sky radiation as a background reflection. This method does not require heating the specimen, therefore the specimen can be measured while keeping it at the normal temperature.