Tacrolimus, an immunosuppressive drug has a variable pharmacokinetic and poor oral bioavailability due to poor solubility. The aim of the present study was enhancing the solubility and oral bioavailability of this drug.Tacrolimus nanoparticles were prepared by precipitation anti-solvent evaporation method. The effect of different parameters including: surfactant type, solvent to nonsolvent ratio and drug to surfactant ratio were studied on the particle size, saturated solubility and dissolution rate of the drug. The solid state characterization was done by fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD) and scanning electron microscopy. The untreated drug (5 mg.kg-1day-1), nanoparticles or physical mixture of the drug and the stabilizers were administered to rats and blood levels of tacrolimus were assessed by electrochemiluminence method. The oral drug administration was done for 10 days then the changes in white blood cells (WBC) and percent lymphocyte count were determined before drug administration, 5 and 10 days after drug administration.FTIR spectroscopy showed no interaction between the drug and stabilizers. XRPD and DSC studies indicated the amorphous state of the drug in nanosuspensions. The solvent to nonsolvent ratio of 1:20 and drug to surfactant ratio of 1:3 enhanced 185 fold the saturated solubility and 17 fold dissolution rate of the drug. In vivo studies also showed tacrolimus nanoparticles significantly reduced the lymphocyte and WBC, enhanced 66 and 34 fold the AUC0-24 and Cmax of the drug, respectively.The precipitation anti-solvent evaporation is a nano-crystalization technique which showed to be an effective approach for enhancing water solubility and bioavailability of tacrolimus.