Stress is a property of thin film material. It's important to develop in-process observation and control it accurately. An instrument for real-time stress measurement based on optical deflection of two parallel light beams was presented. Combined with the crystal monitor, we can observe the evolution of thin film stress in process. This instrument was applied to observe the process of deposition of HfO2 film. The results show that HfO2 films have a stable tensile stress of 360～660 MPa for the given deposition conditions. The tensile stress becomes larger when the vacuum or the temperature of substrate is higher. The stress was affected intensively by the substrate surface, and was stable with the increment of thin film thickness in high vacuum conditions.