YBCO bulks with submicron Y2BaCuO5 particles have been prepared under oxygen partial pressures of 0.21atm and 1atm, respectively, by the powder melting process (PMP). The microstructures, oxygen contents and critical current densities of the samples have been investigated. The results showed that the sample grown under P(O2) = 1 atm contained Y2BaCuO5 particles with a little bigger average size and had a slightly higher density of stacking faults than the one prepared under P(O2) = 0.21 atm. Oxygen content measurements indicated that the as-grown sample under P(O2) = 1 atm had a higher oxygen content. After post-annealing in flowing oxygen, the oxygen content of the P(O2) = 1 atm sample was still higher than that of the P(O2) = 0.21 atm sample. They were examined to be 6.94 and 6.88 with X-ray diffraction for the two oxygenated samples, respectively. Magnetization measurements have been performed on the two types of oxygenated samples and critical current densities (Jc) have been calculated based on the extended Bean critical state model. The P(O2) = 0.21 atm sample exhibits Jc values of 5.47×105 A/cm2 and 2.22×104 A/cm2 in 1 T magnetic field at 20K and 77K, respectively. The P(O2) = 1 atm sample, meanwhile, has Jc values of 9.46×105 A/cm2 and 1.47×104 A/cm2 in 1 T magnetic field at 20K and 77K, respectively. Moreover, at 20K, the Jc of the P(O2) = 1 atm sample is higher than that of the P(O2) = 0.21 atm sample in an applied magnetic field up to 5 T. At 77K, however, the Jc of the P(O2) = 1 atm sample drops more quickly than that of the P(O2) = 0.21 atm sample. The mechanisms of the different microstructures, oxygen contents and magnetic field dependence behaviors of Jc for the two kinds of samples have been discussed.