The roles of adsorbed oxygen and lattice oxygen on the surface of VPO catalyst for n-butane selective oxidation were experimentally studied by using on-line mass spectroscopy (MS) transient response measurement in a fixed bed micro-reactor. The results showed that the lattice oxygen led to selective oxidation，while adsorbed oxygen led to deep oxidation. Gaseous oxygen could be adsorbed on the surface of the VPO catalyst and transformed to lattice oxygen but did not directly participate in selective oxidation. The diffusion of lattice oxygen from the bulk phase to the surface might be the rate-controlling step of unsteady state oxidation when n-butane selective oxidation and VPO catalyst re-oxidation were performed separately.Increasing the diffusion rate of the lattice oxygen and usable lattice oxygen capacity in VPO catalyst could remarkably improve the reaction performance in a fixed bed reactor.