A dosimetric study compared flattened filter (FF) and unflattened filter-free (FFF) 18 MV medical linear accelerators (LINAC) using BEAMnrc Monte Carlo (MC) calculations and experimental measurements. BEAMnrc MC simulations were initially validated against experimental measurements for an 18 MV FF LINAC, with parameters such as the percentage depth dose (PDD) and beam profile measured and calculated per the International Atomic Energy Agency (IAEA) dosimetry protocol TRS 398. Following the validation of the LINAC and water phantom models for MC simulations, BEAMnrc MC calculations were performed to compare the FF and FFF 18 MV LINAC parameters. The results indicate that the BEAMnrc MC accurately simulated the LINAC model, with PDD uncertainties within 2%. Beam flatness differences between the MC simulations and measurements in the plateau region were within 3% and within 2 mm in the penumbra region. The PDD data show that the 18 MV FFF beam delivered a higher dose rate in the buildup region than the FF beam, while beam profile measurements indicate lower out-of-field doses for FFF beams, especially in the 20 × 20 cm² field. These findings provide crucial dosimetric data for an 18 MV FFF LINAC, which is useful for quality assurance and beam matching, and offer a methodology for quantitatively comparing the dosimetry properties of an individual 18 MV FFF LINAC to reference data.