Some of the major 'optics' characteristics of the main beams of PLANCK High Frequency Instrument (HFI) are discussed. The HFI consists of 36 corrugated horns feeding cryogenically cooled bolometric detectors. The optimal design gives angular resolutions and beam lines of sight in agreement with the requirements. The very complex process implied in the precise modeling of the high frequencies channels of PLANCK-HFI is shown.