Nanocrystalline diamond microscopic structures (5 μm width) are grown directly on Si/SiO2 substrates by patterning of a nucleation layer using photolithography prior to the diamond growth. The diamond in a thickness of 300 nm is grown by a microwave chemical vapor deposition on the nucleation patterns. Morphology and material composition of the microstructures are characterized by scanning electron microscopy and atomic force microscopy. The diamond microstructures exhibit clear transistor characteristics in both solid-state and solution-gated field-effect transistor configurations. The conductivity is generated by a transfer doping of hydrogen-terminated diamond surface. Gating of the transistors is realized by a deposition of Al electrode or by an immersion into a pH buffer in contact with Ag/AgCl electrode. The solution-gated field-effect transistors are sensitive to pH.