In this report, we analyzed field factors, which are important parameter in dose calculations, for ⁶⁰Co irregularly shaped fields. Measured field factors for irregular fields were different from the field factors for equivalent square fields obtained from the standard field factor curve measured on square fields defined by installed collimator. They were almost settled on field factors for their open fields. From the result of this experiment, we consider that in dose calculations for irregular fields it is better to use field factors not for equivalent square fields but for the open fields.