Roughening behavior of the free surface of polycrystalline iron during plane strain compression is investigated experimentally. The changes in the shape of the free surface, which is roughened during plastic deformation, are observed in the three-dimensions. It is round that the mountains and the valleys of l~he roughened shape tend to elongate in the constrained direction for the specimen with isotropic grain shape. The shapes of the roughness curves in the loading direction and in the constrained direction are compar,zd. The normalized height distribution of the roughness curve in the constrained direction is symmetric, while that of the roughness curve in the loading direction is asymmetric and positively skew during plane strain compression. Based on a simple simulation of the roughness curves by a random midpoint displacemen! method, this difference is supposed to be caused by the constraint of the material flow under phme strain condition.