In-situ growth of YBaCuO thin films has bee performed in the UHV E-beam coevaporation system with careful control of the evaporation rates of metal based on electron impact emission rate monitors, which enables systematic investigation of the compositional dependence of the quality of grown films. The results show that best quality films can be obtaine in a well defined and narrow compositional range, suggesting that stringent rate control is essential for MBE growth of oxide superconductor films.