Abstract: Interpretation of 40Ar/39Ar ages depends on the closure temperatures of Ar in minerals. Cooling ages are only restricted within the time range of occurrence of an event of metamorphism-deformation and cannot represent the real time of occurrence of a geological event. Therefore we must get the deformation ages. Proceeding from the concept of closure temperatures, this paper discusses the influences of such parameters as pressure, cooling rate, effective diffusion dimensions and geometry, grain sizes and dynamics on the closure temperatures of minerals, and introduces the new concept of "closure window". Based on these influences, the paper also discusses how to get deformation ages.