MgB2　superconductors　were　prepared　by　PIT　method　under　different　conditions.　The　phases　and　microstructure　of　the　samples　were　investigated　by　X-ray　diffraction　(XRD)　and　scanning　electron　microscopy　(SEM).The　curves　of　magnetic　flux　penetration　depth　vs.　alternating　magnetic　field　(bac　vs.　λ＇curve)　of　MgB2　samples　were　measured　by　Campbell　method,　and　the　samples　critical　current　density　in　different　magnetic　fields　were　obtained.　The　results　show　that　coarse　grains　are　obtained　in　MgB2　superconductors　sintered　for　longer　time,　with　is　beneficial　to　electrical　conduction,　however,　the　current　is　reduced　in　magnetic　field　for　the　weaker　pinning　effect.　The　oxidation　layer　at　grain　boundaries　is　thicker　for　the　long-time　sintered　MgB2　bulks,　and　its　superconductor　current　density　is　reduced.