Disclosed are a method and apparatus for determining an intra prediction mode, and for encoding/decoding video using the same. An apparatus for determining an intra prediction mode is provided, the apparatus comprising a calculator to calculate a variation of adjacent pixels to a current block; a threshold setter to set a threshold to evaluate the pixel variation; a comparator to compare between the variation and the threshold; a first prediction mode determination unit to determine a single predetermined prediction mode to be an optimal prediction mode if the variation is below the threshold; and a second prediction mode determination unit to determine a particular prediction mode from a plurality of prediction modes to be the optimal prediction mode through a rate-distortion optimization by performing intra predictions in the plurality of prediction modes if the variation is greater than or equal to the threshold.