To provide a water-based ink excellent in abrasion resistance and color reproducibility, and to provide an ink-jet image forming method using the same.

The water-based ink has a dispersion containing colored fine particles, in which the degree of variation 1 of the volume-based average particle sizes of the colored fine particles before and after stayed at 40°C for 7 days is represented by expression (A), and the degree of variation 2 of the volume- based average particle sizes of the colored fine particles before and after stayed at 80°C for 7 days is represented by expression (B), wherein expression (A) is 0.7 ≤ the degree of variation 1 (volume-based average particle sizes stayed at 40°C/volume-based average particles size before stay)≤1.3 and expression (B) is 0.98 ≥ the degree of variation 2 (volume-based average particle sizes stayed at 80°C/volume-based average particle sizes before stay) or 1.05 ≤ the degree of variation 2.