PURPOSE: The accurate dosimerty of independent collimator equipped for 6MV and 15MV X-ray beam was investigated to search for the optimal correction factor. METHODS AND MATERIALS: The field size factors, beam quality and dose distribution were measured by using 6MV, 15MV X-ray. Field size factors were measured from 3X3cm2 to 35X35cm2 by using 0.6cc ion chamber (NE 2571) at Dmax. Beam qualities were measured at different field sizes, off-axis distances and depths. Isodose distributions at different off-axis distance using 10X10cm field were also investigated and compared with symmetric field. RESULTS: 1) Relative field size factors was different along lateral distance with maximum changes in 3.1% for 6MV and 5% for 15MV. But the field size factors of asymmetric fields were identical to the modified central-axis values in symmetric fileld, which corrected by off-axis ratio at Dmax.2) The HVL and PDD was decreased by increasing off-axis distance. PDD was also decreased by increasing depth For field size more than 5X5cm2 and depth less than 15cm, PDD of asymmetric field differs from that of symmetric one (0.5~2% for 6MV and 0.4~1.4% for 15MV).3) The measured isodose curves demonstrate divergence effects and reduced doses adjacent to the edge close to the flattening filter center was also observed. CONCLUSION: When asymmetric collimator is used, calcutation of MU must be corrected with off-axis and PDD with a caution of underdose in central axis.