This paper proposes a robust inclinometer system using three monaxial microelectromechanical systems accelerometers and three monaxial fluxgate sensors. By formulating a basic three sensitive axes sensor model, we calibrate tilt and azimuth directly through a simple and effective linear model. To improve the accuracy of the proposed model, we present two different optimal solutions to minimize the systematic error, and adopt the interior-reflective Newton method and the sequential quadratic programming method to solve the problems, respectively. Experimental results demonstrate that our system performs excellently with the maximum error of tilt angle 0.09° in our applied measurement range (0°-120°), and the maximum error of azimuth angle 0.4° in the measurement range (0°-360°).