The attenuated total reflection (ATR) properties of C15TCNQ Langmuir-Blodgett (LB) ultrathin films deposited on the Al thin films about 20 nm thick were measured in order to evaluate the actual structure. The C15TCNQ LB ultrathin films were prepared by the vertical dipping method, using two deposition processes. The dielectric constants and the thicknesses of these deposited LB films were theoretically calculated from the ATR curves, taking into account the presence of the natural oxide layer on Al. It was considered that Z-type LB films were obtained when the deposition of each monolayer was carried out only as the substrates were withdrawn. In contrast, when monolayers were deposited during both upward and downward movements of the substrates, the resulting LB films were considered to be not perfect Y-type, but partially Z-type.