PURPOSE: To easily obtain mono-ortho-tertiary butyl-aniline in a high conversion ratio with high selectivity by using a silicaalumina catalyst to react aniline and isobutylene in a specific ratio under a specific reaction condition within a fixed catalyst bed reactor.

CONSTITUTION: A silica-alumina catalyst containing 70-80 wt.% of silica is disposed in a fixed catalyst bed reactor and 1 mol of isobutylene and 2-10 mol of aniline are reacted at 130-190°C, pref, 150-170°C under pressure of 500-1500 psig while a spatial speed (LHSV) is held to 0.05-0.5 hr-1, pref., 0.1-0.25 hr-1 to obtain the objective compd. In order to increase the catalytic activity of silica-alumina, by removing alkali (alkaline earth) metal oxide from the catalyst system, the catalyst can be activated and this activation is performed by bringing the catalyst into contact with an acid to recover the undissolved silica- alumina catalyst.