In formulas employed for analysis of organic electronic devices, the relative dielectric constant value of the semiconductor organic films is often assumed rather than measured, even though it is a fundamental parameter for a correct interpretation. This is particularly true for ultrathin films made of discrete molecular layers. In this work, Spectroscopy Ellipsometry and Scanning Capacitance Microscopy were used to study thin films made of N,N'-bis(n-octyl)-x:y,dicyanoperylene-3,4 : 9,10-bis(dicarboximide). The relative dielectric constant presents a non-monotonic trend with thickness: it is equal to 2.1 for one molecular layer, saturating at 3.2 for increasing thickness. This maximum value, equivalent to the bulk one, occurs when the coverage is in between the third to the fourth layer. In this range, the growth switches from a Frank-Van der Merwe (2D growth) to a Volmer-Weber mode (3D growth); in addition, the molecular configuration assumes a bent/distorted geometry with respect to the initial edge-on one. These results establish a morphological dependence of the dielectric constant, especially in the vicinity of the substrate interface, that disappears at a certain distance from it.