Orientation changes are important factors when considering microstructural changes in materials caused by plastic deformation because these are related to the arrangement and density of dislocations. The three elements of a logarithm of a rotation matrix are called log angles, which compose a rotation vector that can be used as a parameter to represent crystal orientations. Herein, approximate expressions of a rotation vector are derived for a product of two small-angle rotations. The degrees of the first-order, second-order and third-order approximations are discussed. The non-commutativity of two rotations is graphically illustrated using the approximate expressions. Experimental results of orientation changes in a cold-rolled Cu bicrystal are considered to evaluate the validity of the approximate expressions. It is shown that a linear approximation for the product of rotation vectors is valid to treat small-angle orientation changes in plastically deformed crystals with dislocation structures.