A new generalized isothermal equation of state for liquid is obtained from the law of corresponding state. Only one temperature-independent parameter is needed for the density calculations from triple point up to the critical point for both nonpolar and polar liquids including strongly associating liquids such as alcohols and water. The parameter has an obvious physical significance and it can be obtained from generalized correlations. The equation can be applied to polymers also if an isothermal compressibility datum or one density value is known.