Effect of sodium bicarbonate and pHmax buffer supplement on milk fat synthesis under milk fat depression conditions, using of 24 Holstein lactating cows were investigated in a completely randomized block design. Milk fat depression was induced in all cows for 10 d by feeding a diet containing high levels of starch. The experiment was conducted in two periods including: first period (day 1 to 20), allocation of cows to the treatments 1) control, 2) basal diet + 0.8% pHmax and 3) basal diet + 0.8% sodium bicarbonate and in the second period (days 21 to 30), all cows were fed the control diet. Buffer supplements increased milk fat concentration, without affecting dry matter intake, milk yield, and milk protein and lactose concentrations. The concentration of total trans fatty acids decreased in the experimental treatments compared to the control. The trans -10: trans -11 ratio was lower in cows receiving the buffer supplements compared with cows receiving the control, which indicating the effect of these compounds on the improvement of milk fat synthesis by stimulating normal fatty acid biohydrogenation pathways in the rumen. Addition of buffer supplements increased milk fat synthesis, which persisted for one week after cessation of mentioned compounds. Therefore, using of sodium bicarbonate and pHmax can maintain milk fat in dairy cows fed with diets containing high starch and under milk fat depression conditions. Also, using of pHmax reduces the costs of using common buffer compounds such as sodium bicarbonate.