A model for the analysis of X-ray 00l reflections from Langmuir-Blodgett (LB) films of Y-type structure is presented. In the model, the stoichiometrically minimum neutral unit is considered a structural unit for the X-ray analysis. Our treatment proceeds as follows. The geometrical parameters of the structural unit are optimized by a semiempirical molecular orbital calculation. Next, the direction of the chain orientation with respect to the laboratory frame is determined so as to minimize the discrepancy between the calculated and observed structure factors. The analytical scheme was applied to the diffraction from a LB film of cadmium stearate (CdSt(2)), and shown to be valid for determination of the longitudinal structure. The stearate chain was revealed to stand normal to the layer surface within several degrees of tilt. Signs of all the structure factors observed from the CdSt(2) LB film were shown to be positive, and its electron density distribution was obtained by Fourier synthesis. For Y-type LB films of divalent metallic salts of stearic acid, the effect of the metal atom on the signs of the structure factor is discussed.