Being unbound to the state space size, mechanical theorem proving is an important method in ensuring software's correctness and avoiding serious damage from program bugs. File comparison algorithms constitute a large family of algorithms which find wide range of application domains including bio-informatics, information retrieval and network security. This paper presents a work on formalization of fcomp, an efficient line oriented file comparison algorithm suggested by Miller and Myers in 1985, in the interactive theorem prover Isabelle/HOL. A small bug in fcomp's bound variable iteration is identified, and the termination and correctness of the modified algorithm is established. Formal analysis of time complexity is also performed which coincides with the algorithm designers' own results. The presented work lays a valuable foundation for subsequent formal checking of other file comparison algorithms.