A theory for an approximate description of the breakup of a thin liquid jet issuing from an orifice into drops is devised on the basis of mass and energy balance equations. Attention is focused mainly on the effects of the surface tension forces. The mathematical analysis performed permits unequivocal estimation of the relative value of the mean diameter of the drops formed and the distances between them, which agree closely with the known results of experimental observations and the value of the Rayleigh wavelength from the linear theory of jet instability.