Studies on organic fluorine compounds in our laboratory are summarized. New methods for the synthesis of fluorine compound are as follows ; 1) fluorination of alcohols with fluorophosphoranes, 2) trifluoromethylation of aromatic and aliphatic halides with trifluoromethyl halides and copper powder, and 3) cycloaddition reaction with novel fluorine reagents like hexafluorobutyne. Reactions of trifluoromethyl aromatic compounds were investigated thoroughly, and some interesting characters of fluorine compounds were revealed. Trifluoromethylated valence bond isomers of aromatic compounds were synthesized, and some novel reactions of these highly strained compounds were shown. The characteristics that a fluorine atom is a small substituent and that a carbon-fluorine bond is very strong were used to study the metabolism of vitamin D3. Some interesting reactions by use of fluorine compounds as synthetic reagents are also discussed.