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Three types of boron precursor powders with a purity ranging from 90% to 99% from Alfa Aesar were investigated. A common magnesium powder was mixed with each type of the boron powders in a stoichiometric MgB2 composition. The samples were heat-treated at 700 deg C or 850 deg C for 30 min in vacuum or in flowing Ar atmosphere. The results show the samples annealed in vacuum have a higher Jc when annealed at a lower temperature. On the contrary, samples annealed in the flowing Ar gas showed a higher Jc when annealed at a higher temperature. At a particular annealing temperature, samples annealed in the Ar atmosphere always showed a higher Jc compared to vacuum annealing. One of the reasons is attributed to a greater magnesium loss when annealed in vacuum and at high temperatures.