Studies were carried out for the purpose of production of C9, C10 aromatic hydrocarbons by catalytic reforming of heavy naphtha.1) Naphtha fraction with the boiling range from 100°C to 210°C, which was obtained from Gach Saran crude, was further fractionated into fractions, each having a 10°C boiling range. These naphtha fractions were then reformed, using a platinum-alumina commercial reforming catalyst. It was confirmed by the experiments that the fraction ranging from 130°C to 170°C and from 150°C to 205°C gave higher yields of C9, C10 aromatics, respectively. In addition, the relationship between the boiling range of naphtha and the yields of methylethybenzene (MEB), trimethylbenzene (TrMB), and tetramethylbenzene (TeMB) was also investigated. Components of these three isomers-i.e. MEB, TrMB, TeMB-in the reformed naphtha remained constant without regard to the boiling range of the feed naphtha.2) Straight run Gach Saran naphtha (125∼190°C) was reformed at the temperature 450∼530°C, the pressure 10∼50atm., the lipuid hourly space velocity 0.6∼4 and the mole ratio of hydrogen to hydrocarbon 5∼15. From the data obtained, the relationship between the operating condition and yields or compositions of C6∼C11 aromatic hydrocarbons, particularly MEB isomers, TrMB isomers, TeMB isomers and durene fraction in those reformates was investigated.