Provided is a catalyst composition for the isomerization of n-paraffin which comprises a .beta.-zeolite having been synthesized according to a dry gel process and contains cerium, the .beta.-zeolite being characterized in that it has an average particle diameter of 0.01.about.0.1 .mu.m and the proportion of the cerium in the .beta.-zeolite is within the range of 0.001.about.0.2 in terms of the atomic ratio of the silicon atom in the .beta.-zeolite to the cerium atom, i.e. [Ce]/[Si] as well as a process for isomerizing n-paraffin which comprises bringing n-paraffin into contact with the catalyst composition as set forth above.