The effect of different crust-mantle mixing environments on Rb-Sr isochron ages is discussed in terms of a two-component mixing model. Normally, the mixing of mantle magma with crust material will result in an older isotopic age than the crystallization age of a magmatic massif. If the crust material experienced a Rb-lost or/and Sr-gained event, the slope of the isochron will vary greatly depending on the isotopic ratios of the two components. The slope may result in a very old age and abnormally low initial isotopic ratio, sometimes even a negative slope. If the mixing was done through separate diapirism, the crystallization age for the series of small diapirs can be derived by the internal mineral isochron of a single small diapir, and the age of the country rock may also be derived by the whole-rock isochron of different diapirs (assuming the same mixing proportions for every diapir). The present study can shed light on some confusion in interpreting Rb-Sr isochron ages.