The emissivity of the blackbody is a very important parameter in spectral radiance measurement systems. In the conventional method, the emissivity is calculated based on the isothermal model. However, the actual temperature distribution in the blackbody cavity is always nonisothermal; the emissivity calculated based on the isothermal model may not accurately present the radiation characteristic of the blackbody. In this study, the actual temperature distributions of two blackbodies (one has an extended cone shape, and the other a 65-mm diameter cylindrical shape) are measured, and the emissivities are calculated accordingly based on the nonisothermal model at a certain temperature (873 K). The results show there are different tendencies of temperature distributions in the two blackbodies. When compared with the isothermal model, the emissivities in the 873 K temperature and 2.0-20.0 µm wavelength condition are about 1.75% and 0.18% lower at the nonisothermal model for the extended cone shape and cylindrical blackbodies, respectively. To improve the emissivity, different types of apertures are customized for the two blackbodies. For the extended cone-shaped blackbody, the emissivity in the 873 K temperature and 2.0-20.0 µm wavelength condition increases by 1.12% when using a ring-shaped graphite aperture in the cavity, whereas for the cylindrical-shaped blackbody, the emissivity in the same condition increases by 0.09% when using a high-reflective aperture in front of the cavity opening. Different from previous studies, this study provides new insight in calculating and improving the effective emissivity of blackbodies by using the measured temperature in the cavity based on the nonisothermal model.