Abstract: Two luminescence phases were observed in annealed CsI:Pb crystals at the 10~30 K temperature range, which are dominant emission round 464 nm with the excitation at 410 nm and dominant emission round 422 nm with the excitation about 360 nm and 300 nm, respectively. It was found that their relative intensities depend strongly on the annealing time and the origin of the emission bands and the formation mechanism of different Pb²⁺-based aggregates is discussed.