The correlation between the intergranular corrosion (IGC) behavior and the aging temper (including T6 aging at 150 and 175 ℃, T8 aging at 150 ℃) of 2099 Al-Li alloy was investigated. Then a diagram was established to describe the relation of IGC with the aging procedure. According to the aging resulted corrosion morphology of the alloy, the aging process might be divided into four stages: i.e. stage I is the initial aging stage, stage II the under-aging stage, stage III the near-optimal aging stage and stage IV the over-aging stage. The aged alloy showed different modes corrosion after aged by different stages: i.e. pitting corrosion or local intergranular corrosion (LIGC) for stage I; general intergranular corrosion (GIGC) for stage II; LIGC for stage III; and pitting corrosion without IGC for stage IV. In the under-aged alloy, a great number of very fine precipitates preferentially deposited along grain boundaries, which seem like a continuous chain, hence resulting in the highest susceptibility to IGC for the alloy. In the over-aged alloy, the precipitates were coarsened and the inter-precipitate spacing became larger, so that the susceptibility to IGC may be suppressed. In general, with the rising of aging temperature, the grain boundary precipitates grew coarse and the spacing between the precipitates became larger, while a proper pre-deformation of the alloy might facilitate the uniform nucleation of precipitates and suppress the formation of precipitate-free zone (PFZ) along grain boundary. These two factors shortened or even eliminated the Stage II and Stage III, therewith increasing the resistance to IGC of the alloy.