Surface sturctures of GaAs(001) under growth conditions have been investigated using in situ X-ray diffraction. Theatomic arrangements of GaAs(001) surface stabilized at an elevated temperature under As pressure were quantitativelydetermined in a molecular-beam epitaxy chamber integrated with an X-ray diffractometer. With the help of high angularresolution of synchrotron radiation, disordered sructures appearing in the transition from (2×4) to other phases wereclarified. Energy tunability of synchrotron X-rays allowed for element-sepcific analysis of the c (4×4) struture,providing direct evidence for Ga-As heterodimer formation.