Recent progress of cyclotron ion beam development for applications in materials science and biotechnology at the ion‐irradiation research facility TIARA of the Japan Atomic Energy Research Institute(JAERI) is overviewed. The AVF cyclotron in TIARA can accelerate protons and heavy ions up to 90 MeV and 27.5 MeV/n, respectively. In order to conform to the requirement of a reliable tuning of microbeam formation, the cyclotron beam current has been stabilized by controlling the temperature of the magnet yoke and pole within +/−0.5° and hence by decreasing the variation of the magnetic field &Dgr;B/B below 10−5. A heavy ion microbeam with energy of hundreds MeV is a significantly useful probe for researches on biofunctional elucidation in biotechnology. Production of the microbeam with spot size as small as 1&mgr;m by quadrupole lenses requires the energy spread of the beam &Dgr;E/E < 2 × 10−4. In order to minimize the energy spread of the cyclotron beam, the fifth‐harmonic voltage waveform has been successfully superposed on the fundamental one to make energy gain uniform. © 2003 American Institute of Physics