Provided are a method and an apparatus for determining an intra prediction mode. The method includes: splitting a maximum coding unit into coding units of a first size; acquiring an edge direction of each of the coding units of the first size; grouping adjacent coding units of the first size based on uniformity of edge directions of the adjacent coding units of the first size; determining a size range of coding units to be applied to intra prediction on the intra prediction coding unit group; performing the intra prediction on the intra prediction coding unit group by using coding units having sizes included in the determined size range; and determining a coding unit for the intra prediction and an intra prediction mode of the coding unit by comparing costs according to the performed intra prediction on the coding units having the sizes included in the size range.