We propose an accurate and easy-to-implement method on rotation alignment of a camera-inertial measurement unit (IMU) system using only a single affine correspondence in the minimal case. The known initial rotation angles between the camera and IMU are utilized; thus, the alignment model can be formulated as a polynomial equation system based on homography constraints by expressing the rotation matrix in a first-order approximation. By solving the equation system, we can recover the rotation alignment parameters. Furthermore, more accurate alignment results can be achieved with the joint optimization of multiple stereo image pairs. The proposed method does not require additional auxiliary equipment or a camera's particular motion. The experimental results on synthetic data and two real-world data sets demonstrate that our method is efficient and precise for the camera-IMU system's rotation alignment.