The effects of the various preparation procedures of the Dragendorff reagent on sensitivity for thin layer chromatography (TLC) were examined. The sensitivity to various compounds was different depending on the preparation procedures of the Dragendorff reagent.   The reagent with the highest sensitivity to most of the compounds tested was the one prepared with bismuth subnitrate and concentrated hydrochloric acid. However, color spots were disappeared relatively in a short time after the reagent was sprayed on the compounds. The reagent with the second highest sensitivity was the one prepared with precipitation of Bi(OH)₃ from bismuth subnitrate, although the procedure was complicated and time-consuming.   Consequently, to simplify the preparation procedure of the reagent, we modified it to the procedure without precipitation of Bi(OH)₃ from bismuth subnitrate. The reagent was also prepared from commercially-manufactured Bi(OH)₃ or BiI₃. The modified Dragendorff reagents showed almost the same sensitivity to most of the compounds tested as the one prepared with precipitation of Bi(OH)₃ from bismuth subnitrate, and would be useful for practical TLC analysis.