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).