This paper explores a quaternion based extended Kalman Filter (EKF) for estimating the attitude of a nonholonomic spherical robot using a 3-axis gyroscope, accelerometer and magnetometer. A low cost inertial measurement Unit (IMU) and magnetometer are mounted on the spherical robot and the measured data are fused with EKF to determine the attitude of the robot. The attitude of a spherical robot is a time-parameterized curve in SO(3) and hence is ideal for validating its attitude globally. An indoor experiment was carried by dead-reckoning on circular and trifolium trajectory. The ground truth was established by integrating the robot kinematics using the estimated attitude and then comparing it with the reference trajectory. A high cross correlation between the experimental data and true trajectory was obtained suggesting a strong match. © 2019 Association for Computing Machinery. All rights reserved.