Abstract: This research was aimed to examine the effect of Rumen Undegradable Protein (RUP) supplementation on estradiol profile and estrous behaviour in heifers maintained in tropical climate. A total of 36 Brahman Cross (BX) beef heifers were divided into 2 groups, control group (ration without supplemented RUP) and treatment group (ration with supplemented RUP). The heifers were inseminated on third days after ovaries checking with USG and synchronized with PGF2α . Blood sampling was conducted to measure the estradiol concentration in 5 points: during the estrous (0); 5; 17; 19 and 21 days. Estrous behaviour were recorded 4 times a day. Results showed no significant effect (p>0.05) of feeding RUP between control and treatment group to estradiol concentration (130.96±40.96; 134.75±39.00; 137.31±47.24; 133.48±36.90; 143.89±55.83 pg mL-1 vs. 126.59±34.86; 124.93±38.82; 123.20±42.39; 126.11±46.45; 126.25±32.66 pg mL-1) and estrous behaviour but there was a significant correlation (p<0.01) between estradiol concentration to estrous behaviour. In conclusion, the level of estradiol concentration and estrous behaviour in heifers were not effected by undegradable protein supplementation.