It is commonly accepted that gate dielectric dipoles can induce energetic disorder in organic field-effect transistors. However, convincing experimental evidence that directly demonstrate this effect are still in lack. In this work, we present a combined experimental and theoretical study to reveal this effect. We have investigated the temperature-dependent mobility of two rubrene single-crystal transistors with different polymer dielectrics. Model fittings of the data indicate there is higher energetic disorder in the device on dielectric with larger permittivity. Scanning Kelvin probe microscopy was then employed to directly characterize the density of tail states, which is correlated with energetic disorder, in the devices. The results further confirm that dielectric dipoles can increase energetic disorder in organic semiconductors.