We have demonstrated an optical method for measuring the kinetics of the Si(111) phase transition. The system is based on the temporal evolution of the surface second-harmonic generation (SHG) intensity from 7 7, @'1 1@' and their mixture of Si(111) in response to a stepwise temperature change. The temporal evolution of the SH signal around the transition temperature is shown to reflect a delayed response of the 7 7 @'1 1@' phase transition with a response time of ∼ 3 s both for heating and cooling.