The highly exothermic fixed-bed catalytic reactor for the partial oxidation of o-xylene to phthalic anhydride was studied with the objective of analyzing heat transfer phenomena and reaction rate. The conventional problem of highly exothermic fixed-bed catalytic reactor is closely associated with a hot spot which leads to excessive temperature rise due to immoderate oxidation reaction at inlet region of reactor. At some location of axial direction hot spot gives rise to decrease of yield for desired product, unstable operation of fixed-bed catalytic reactor, rapid activity reduction of catalyst at hot spot zone, and explosion of reactor in extreme cases. The heat transfer phenomena in such a highly exothermic fixed-bed catalytic reactor are studied by using heat transfer parameters and the reaction rate which is the essential cause of heat problem is also studied.