In the area of natural language processing, a lot of unification-based grammar formalisms have been issued and are highlighted. The core of the formalisms is an unification of feature stutures. But few formal unification methodologies have been developed. Therfore, implementers of the formalisms could not help adopting an ad hoc design strategy for the unification module. In this paper, we suggest an efficient unification methodology using FUL, a programming language whic supports the unification. For the exeample, we will trace the unification processes in LFG and HPSG, the representative unification-based grammar formalisms, with FUL.