SW China Normal Univ, Dept Phys, Chongqing 630715, Peoples R China
; Acad Sinica, Inst High Energy Phys, Beijing 100039, Peoples R China
The quark model with orbital motion of the valence quarks is constructed to reproduce the spin structure of baryons. The relations between the spin-averaged sum rules and baryon magnetic moments found in the previous works do not remain, unless the small orbital magnetic moments are neglected. In particular, when the orbital motion of the valence quarks leads to the small contribution of quark orbit-spin to baryon magnetic moments, the sum rules for polarized nucleon are in agreement with the recent experiment.