The instability of a viscous liquid sheet issued in an inviscid gas medium is investigated. The dispersion relations between the growth rate and wave number of both symmetric and antisymmetric disturbances are derived and solved numerically. The effects of Weber number, gas-to-liquid density ratio, and Ohnesorge number on the growth rates of two- and three-dimensional disturbances are studied. It is observed that at low Weber number, two-dimensional disturbances always dominate the instability of symmetric and antisymmetric waves. When the Weber number is high, long-wave three-dimensional symmetric disturbances have a higher growth rate than their two-dimensional counterparts, while the opposite is hue for antisymmetric disturbance. For short waves, both two- and three-dimensional disturbances grow at approximately the same rate. Increasing the gas-to-liquid density ratio or decreasing the Ohnesorge number enhances the departure in the growth rates of two- and three-dimensional symmetric disturbances of long wavelength. Both the maximum growth rate and the dominant wave number increase with Weber number and density ratio but decrease with Ohnesorge number.