A fixed-angle rotor, having a centrifuge tube inclined at a predetermined gradient angle with respect to a rotational axis, is used to obtain a sedimentation coefficient of sample particles. In one method, a distance in the centrifugal direction is corrected by the gradient angle .theta. of centrifuge tube 1. Sedimentation coefficient S.sub.20,W is calculated based on the corrected distance, and then the sedimentation coefficient S.sub.20,W is multiplied with sin(.theta.) to obtain a corrected sedimentation coefficient. In another method, a distance in the centrifugal direction is corrected by the gradient angle .theta., and also an angular acceleration .omega..sup.2 is corrected by dividing it by sin(.theta.). And then, the sedimentation coefficient S.sub.20,W is calculated based on thus corrected distance and corrected angular acceleration.