New yellow synergist was synthesized for pigment dispersion in pigment ink. 1,4-Butanesultone was applied to Pigment Yellow 74 (PY74) as a hydrophilic functional group in order to synthesize 4-[{2-[(2-Methoxy-4-nitro-phenyl)-(4-sulfo-butyl)-hydrazono]-1-methylene-3-oxo-butyl}-(2-methoxy-phenyl)-amino]-butane-1-sulfonic acid (PY74ASA-2). Whereas pigment showed extremely low solubility in water and organic solvents such as DMSO, DMF and methanol, the synergist was found to have higher solubility than pigment. This result can be interpreted as reduction of particle aggregation by increasing polarity. This synergist applied to pigment ink is expected to improve dispersion property and storage stability of ink.