This study was conducted to evaluate the effects of top–dressed sodium bicarbonate, alfalfa and wheat straw on dairy cows performance under heat stress condition. A total of 12 early lactation dairy cows with milk production of 43.1 kg/day and lactation number of 2.35±1.39 were assigned to 4×4 latin square design. The experimental diets were: 1) control (basal diet free of top-dress), 2) basal diet+top dress of Sodium bicarbonate, 3) basal diet+top dress of alfalfa, 4) basal diet+top dress of straw. Dry matter intake (DM) obtained 24.75, 24.85, 25.13 and 25.11 kg/d respectivly for control, sodium bicarbonate, alfalfa and straw diets (P 0.05). Digestibility of DM and organic matter (OM) were higher for cows fed contral and top-dress of sodium bicarbonate compared to diets containing top-dress of alfalfa and straw (P 0. 01). The results showed that digestibility of fiber was lower in cows fed and top-dress of straw compared to control diet and sodium bicarbonate (P 0.05). Milk yield tended to be higher in cows fed top-dress of straw, alfalfa and sodium bicarbonate than control (P =0.09). The milk corrected for energy and fat, as well as fat and protein percentage did not affected by experimental diets, while percentage of total solid was lower in cows fed straw compared to control and top-dress of alfalfa (P 0.05). The concentration of saturated fatty acids (SFA) did not affected by experimental diets, while unsaturated fatty acids (USFA) was higher for cows fed top-dress of alfalfa compared to sodium bicarbonate (P 0.05). Eating and chewing time (min/day) tended to be higher for cows fed top-dress compared to control diet (P =0.09). The higher and lower rumen pH were obtained 6.16 and 5.73 for cows fed straw and control diet respectively (P 0.05). The percentage of volatile fatty acids (VFAs) in ruminal fluid, as well as, the concentration of glucose, total protein, albumin, high dencity lipoprotein (HDL), triglycerides and aspartate aminotransferase and alanine aminotransferase were not affected by experimental treatments, while blood cholestrol concentration tended to be higher for top-dressed diets comparet to control (P = 0.09). Based on the results, top-dress of alfalfa promoted eating and USFA in milk, while straw had positive effect on rumen pH and milk yield, and sodium bicarbonate decreased selection behaviour in daity cows. Key Words Top-dress, rumen fermentation, blood parameters, milk yiled, sodium bicarbonate, alfalfa, wheat straw.