A new equation is derived correlating experimental results for steam by Nobbs-Mayhew, and for refrigerant 21 by Kutateladze et al., concerning condensation heat-transfer on horizontal tube bundles. The method of deriving the equation is an extension of the traditional method, which was improved by the authors. The accuracy of the present equation is almost the same as that of McNaught's equation, if adequate relationships concerning two-phase forced convection heat-transfer to respective substances are applied to the latter equation.