Purpose: ((R)-1-(2-chlorophenyl)-N-1-[11C]methyl-N-(1-propyl)-3-isoquinoline carboxamide ((R)-PK11195) is a specific ligand for the peripheral type benzodiazepine receptor and a marker of activated microglia, used to measure inflammation in neurologic disorders. We report here that a direct and simple radiosynthesis of [11C](R)-PK11195 in mild condition using NaH suspension in DMF and one-step loop method. Materials and Methods: (R)-N- Desmethyl-PK11195 (1 mg) in DMSO (0.1 mL) and NaH suspension in DMF (0.1 mL) were injected into a semi-prep HPLC loop. [11C]methyl iodide was passed through HPLC loop at room temperature. Purification was performed using semi-preparative HPLC. Aliquots eluted at 11.3 min were collected and analyzed by analytical HPLC and mass spectrometer. Results: The labeling efficiency of [11C](R)-PK11195 was 71.8±8.5%. The specific activity was 11.8±6.4 GBq/μmol and radiochemical purity was higher than 99.2%. The mass spectrum of the product eluted at 11.3 min showed m/z peaks at 353.1 (M+1), indicating the mass and structure of (R)-PK11195. Conclusion: By the one-step loop method with the [11C]CH3I automated synthesis module, [11C](R)-PK11195 could be easily prepared in high radiochemical yield using NaH suspension in DMF. (Nucl Med Mol Imaging 2009;43(4):337-343)