An ink for ink jet printing which contains as essential components pigment, water-soluble organic solvent, and water. The ink contains particles, after ink jetting, such that the number-average particle diameter is 10 to 150 nm and the number of particles larger than 0.5 &mgr;m is 6×104 to 6×105/&mgr;l and the number of particles larger than 5 &mgr;m is less than 50/&mgr;l. This ink may be used in combination with a second liquid which contains a water-soluble organic solvent having a solubility parameter of 9 to 11, and/or surfactant, and water. A method of ink jet printing uses the ink or a combination of the ink and the liquid. The ink possesses the characteristics of both optical density and storage stability. The liquid prevents the mixing of colors of adjacent printed images.