The present invention relates to a radiation amount estimating method, and more particularly, to a radiation amount estimating method of an MRI based composite image using a lookup table. To this end, the object of the present invention is to provide a radiation dose estimating method of an MRI based composite image using a lookup table including: generating a lookup table representing a linear attenuation coefficient of a region of interest from a CT image (S100)； photographing at least two of a T1 image, a T2 image, and a PETRA image of a patient from MRI equipment (S120)； generating a composite image by superimposing the photographed images (S140)； representing a border line of the region of interest in the composite image to distinguish the region of interest (S160)； designating the linear attenuation coefficient of the lookup table in the distinguished region of interest (S180)； and estimating a radiation dose based on the composite image to which the linear attenuation coefficient is designated (S200).
