Self-assembled monolayers (SAMs) of alkyl silane compounds have been used for modifying gate dielectrics surface of organic field-effect transistors (OFETs) and they have frequently shown improvement of FET performances. In this paper we deposited alkyl silane SAMs by simple spin-coating technique onto Si/SiO₂ substrates. Spin-cast octadecyltrimethoxysilane (OTMS) SAMs had ultra smooth crystalline surface and provided an excellent dielectric surface for OFETs. In fact on the OTMS SAM treated dielectric, pentacene OFETs showed hole mobilities over 2.0 cm²/Vs and electron mobilties over 1.0 and 5.0 cm²/Vs were demonstrated for 3,4:9,10-perylene diimide derivative and C60, respectively. Fabrication technique and characterizations of the OTMS SAMs is described.