Density p, refractive index n(D), and viscosity eta for ionic liquids [C(n)mim]Cl (C(n)mim = 1-alkyl-3-methylimidazolium; n = 2, 4, 6, 8 for ethyl, butyl, hetyl, and octyl) + N,N-dimethylformannide (DMF) binary systems have been investigated at atmospheric pressure from 288.15 K to 318.15 K. The molar excess Gibbs energy Delta G*(E), volumetric properties including excess molar volume V-E, and apparent molar volume V-psi,V-i have been calculated from the experimental data. Furthermore, the refractive index and viscosity deviations (Delta n(D), Delta eta) from the ideal behavior have been obtained and well fitted to the Redlich-Kister equation. Our results show that DMF can effectively adjust the physicochemical properties of 1-alkyl-3-methylimidazolium ionic liquids.