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We investigated on microstructure and superconducting properties of high-density MgB2 superconductors synthesized by a heat-treatment method combining a short period of sintering at 1100degC with a following annealing at a low temperature. The two-step sintering method was found to achieve a coexistence of a high density and broadened XRD full width at half maximum (FWHM) of MgB2 peak in the samples. The onset of diamagnetism at 5 T was approximately 2 K higher than those of the samples heat-treated by single-step sintering at high or low temperatures. Different second-step annealing temperatures were examined in this work. Microstructural analysis indicates that a lower second-step annealing temperature led to further suppression of the large grain growth in the MgB2 samples, which is beneficial to the pinning property of the samples. The energy loss peaks of ac susceptibility measurement in fields confirm that by decreasing the second-step sintering temperature flux pinning of MgB2 in high fields was enhanced.