It has been reported that the addition of citric acid on Co-Mo/B2O3/Al2O3 catalysts by a post-treatment considerably increases the HDS activity to the maximum potential activity compared with a simultaneous impregnation method. With the post-treatment, citric acid consumes already dispersed CoMoO4 and polymolybdate species and thus increases the amount of Co-Mo-S after sulfidation. It seems that the effect of citric acid addition is greater for the calcined Co-Mo catalysts. In the present study, therefore, we investigated effect of calcination temperature of Co-Mo/B2O3/Al2O3 catalysts with the addition of citric acid by a post-treatment. Several characterizations were conducted in order to get the physicochemical properties of the prepared catalysts. It is shown that the HDS activity of the post-treatment Co-Mo/B2O3/Al2O3 is increased up to the calcination temperature of 500 deg. Celsius, the optimum temperature, suggesting that the large amount of well dispersed CoMoO4 is formed while the sintering process occurs with a higher calcination temperature. Moreover, it is found that the HDS activity of Co-Mo/B2O3/Al2O3 catalysts is independent of the pH value on the citric acid addition. Co-Mo/Al2O3 catalysts were also investigated.