Commercial　boron　powder　was　purified　by　a　heat　treatment　method　and　a　chemical　method,　and　the　effect　of　the　differently　purified　powders　on　the　microstructure　and　superconductivity　of　MgB2　was　systematically　investigated.　Both　purification　processes　successfully　decreased　the　oxygen　content,　but　neither　could　completely　eliminate　oxygen　in　the　boron　powder.　The　significant　degradation　of　MgO　peaks　and　increase　in　superconductivity　for　the　sample　with　heat　treatment　purified　boron　powder　indicated　the　best　purification　performance.　For　comparison,　MgB2　bulks　with　chemical　purified　boron　powder　were　also　investigated.　The　reason　of　incomplete　purification　on　the　oxygen　and　the　reaction　mechanisms　for　the　differently　purified　boron　powders　during　the　synthesis　of　MgB2　were　uncovered.　This　research　provides　an　important　foundation　for　understanding　the　reaction　of　oxygen　during　MgB2　synthesis,　and　thus　finally　reveals　the　effect　of　oxygen　on　superconductivity　in　this　system.