For a class of system, the potential of whose Bosonic Hamiltonian has a Fourier representation in the sense of tempered distributions, we calculate the Gaussian effective potential within the framework of functional integral formalism. We show that the Coleman's normal-ordering prescription can be formally generalized to the functional integral formalism.Recently, one of the authors, Lu, with his other collaborators obtained formulae of the Gaussian effective potential (GEP) [1] for a relatively general scalar field theory (see Eq.(1)) in the functional Schrödinger picture [2]. There, the Coleman's normal-ordering prescription [3] was used, and accordingly these formulae have no divergences in low dimensions. Employing these formulae, one can obtain the GEP of any system in a certain class of models, which will be specified below, by carrying out ordinary integrations without performing functional integrations. In this paper, we demonstrate that the same formulae of the GEP can also be obtained within the functional integral formalism. In doing so, we also show that, although quantities in the functional integral formalism are not operators, the Coleman's normal-ordering prescription can be formally used for renormalizing the GEP in the cases of low dimensions. We believe that our simple work is interesting and useful, since the functional integral formalism is importmant in quantum field theory, nuclear and condensed matter physics [4], and can be used for performing some variational perturbation schemes [5,6].In this paper, we first generalize the Coleman's normal-ordering prescription to the functional integral formalism. This formal generalization will be realized by borrowing the normal-ordered Hamiltonian expression in the functional Schrödinger picture because the Euclidean action for a system has the same form with the corresponding classical Hamiltonian in the Minkowski space. Then, following the procedure in Ref.[6], we calculate the GEP for a class of systems. Finishing the above generalization, as an explicit illustration, we will perform a model calculation for the λφ 4 field theory. Consider a class of systems, scalar field systems or Fermi field systems which can be bosonized, with the Lagrangian densitywhere the subscript x represents, x = ( x, t), the coordinates in a (D + 1)-dimensional Minkowski space, ∂ µ and ∂ µ are the corresponding covariant derivatives, and φ x the scalar field at x. In Eq.(1), the potential V (φ x ) has a Fourier reprensentation in a sense of tempered distributions [7]. Speaking roughly, this requires that the integral2 dα with a positive constant C is finite. Obviously, quite a number of model potentials, such as polynomial models, sine-Gordon and sinh-Gordon models, possess this property.For the system, Eq.(1), the conjugate field momentum is expressed as Π x ≡ ∂L ∂(∂tφx) = ∂ t φ x , and the Hamiltonian density is given byIn a time-fixed functional Schrödinger picture at t = 0, one can normal-order the Hamiltonian density H x with respect to any given mass-dimension co...