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Ti-doped MgB2 superconductors with different doping levels were prepared by solid-state reaction at ambient pressure. Jc of the samples changes with the doping level, with the best result achieved at x=0.1. At 5 K, the Jc reaches 2x10(6) A/cm2 in the self-field. In addition, degradation in water of Jc and irreversibility field (Hirr) of MgB2 bulks is significantly reduced by Ti doping. Microstructural analysis reveals that Ti mainly forms a thin TiB2 layer in the grain boundaries of MgB2. At the same time MgB2 grains are greatly refined, forming a strongly coupled nanoparticle structure. Our results show that the unique microstructure of the MgB2 nanoparticles with TiB2 nanograin boundaries may take the responsibility for the enhancement of the flux pinning and the chemical stability against water.