In the carbonization of hydrotreated coal tar pitch containing fine molybdenum and ruthenium particles, a tritium tracer method revealed that prior addition of fine molybdenum and ruthenium particles into the pitch can selectively catalyze the condensation of low molecular weight hydrocarbons at temperatures below 600 degrees C. This is because they prevent the release of those molecules by vaporization and result in an increase in the carbonization yield of coal tar pitch and the hydrogen content of resultant cokes. The carbonization yield with ruthenium particles was higher than that with molybdenum particles. Observations by a transmission electron micrograph showed that the ruthenium particles (10 nm even at 1000 degrees C) were smaller than molybdenum particles and were dispersed uniformly throughout the hydrogenated pitch.