Surface segregation of Sb atoms at low temperatures below 400 degrees C during Si molecular beam epitaxy (MBE) growth is studied by ex situ X-ray reflectivity measurements and secondary ion mass spectroscopy (SIMS). One monolayer of Sb atoms was first deposited at the temperature of 300 degrees C, followed by a 23-nm thick Si overlayer grown at different temperatures of 250, 300, 350, and 400 degrees C. The decay lengths of dopant Sb distribution profiles are obtained to be 0.15, 0.95, 3.5, and >20 nm by simulations of their X-ray reflectivity curves, respectively. A strong surface segregation of Sb atoms is observed at temperatures of 350 and 400 degrees C, which is also confirmed by the SIMS profiles. Surface structure change caused by a high coverage of Sb is suggested to explain such a strong segregation.