Determining the optimal timing is the core of preventive maintenance.Highway agencies always face with the challenge of determining optimal timing for preventive maintenance,particularly in China where there are no condition indicators designed for determining adequate timing for applying preventive maintenance and little literature relating to the development of pavement performance.This work presented the indicators,including crack ratio (Rc),rutting depth (DR),intational roughness index (IIR)and sideway force coefficient (CsF) to determine the adequate timing for preventive maintenance in China.The proper ranges of each indicator to apply to preventive maintenance were then recommended.They are 0.28％-1.4％ for RC,10-15 mm for DR,1.97-3.5 for IIR,40-50 for CSF.Based on pavement condition survey data collected on the test roads in Hebei Province,China,on the application of slurry seal at different timings,the pavement performance was established and the adequate timings for applying slurry seal was studied.Based on benefit-cost analysis,it is suggested that the fourth year is the optimal timing for applying slurry seal based on the condition in China.A framework is established to determine the adequate timings of applying other preventive maintenance methods.