As a prerequisite of biaxial zero creep experiments, a novel sensitive apparatus is developed for real-time filmstress measurement during thermal cycles. The optical sensor with a fixed multi-beam emitter and a CCD detector is investigated during an annealing process of Ag/Co multilayer thin film. The monitoring plots of stress as functions of temperature and time demonstrate the capability of this set-up. The typical sensitivity for measuring the wafer curvature radius is 2km.