In this work, we propose a novel approach for robust localization in slopes using inertial sensors and magnetic information. In environments with slopes, as sensors tilt with the robot when it is moving on the slope, accurate sensor data cannot be correctly measured, reducing localization accuracy. To perform localization using magnetic information, we use information from an inertial measurement unit to estimate the robot's attitude and compute accurate magnetic azimuth information that accounts for attitude changes. Furthermore, pitch information computed from the inertial measuring unit can even be used to enhanced localization, if pitch information from the environment is also collected. By combining these methods with standard geometric landmark localization using 2D laser rangefinders, we have developed a localization system that is robust to the presence of steep slopes, demonstrated by our testing in real environments.