Evaporation of tellurium (Te) occurs during most of the fabrication methods which leads to a strong deviation on thermoelectric properties. Due to the existence of volatile tellurium has been a great challenge to determine the accurate chemical compositions in the Bi0.5Sb1.5Te3 compound for optimizing the carrier concentration. Herein, we systematically verify the reliability and understand the correlations between chemical composition and thermoelectric properties at different portions of large (25 mm), thick (30 mm) samples. The results reveal that spark plasma sintered samples didn’t show any significant evaporation of Te. However, the chemical composition slightly changed, resulting in the final thermoelectric figure of merit at different portions of thick samples varied typically within a 10% error. Moreover, this work supports that the selection of the powder fabrication and compaction method plays a crucial role on the final thermoelectric properties of the material, as the combining of gas atomization and spark plasma sintering can prohibit the evaporation of tellurium and any possible inhomogeneous stress field or temperature distribution in large-sized graphite die to SPS.