Liner and nonlinear-optical properties of oligothiophenes self-assembled monolayers (SAM) are studied by optical second harmonic interference technique. Linear polarizability and first hyperpolarizability as functions of the number of thiophene rings are calculated using a self-consistent approach for linear and nonlinear optical properties. Both of these dependencies can be described by a power law; a nonzero first hyperpolarizability points to the asymmetry of delocalized electron system that might be due to an adsorption of thiophene molecules on a metal substrate. The influence of a dipole-dipole interaction of molecules arranged in the SAM is discussed.