Modified gravity theories are interesting alternatives to describe the accelerated expansion in the early universe, without the need of an extra scalar field. In this work, we discuss the post-inflationary preheating phase in f(R) theory of gravity. We investigate the correspondance between the Einstein and Jordan frame Lagrangian, in order to explain the parametric resonance effect needed for preheating. We consider different forms of potentials in the Einstein Frame and a conformal-like coupling \(\sim R \psi ^2\) in the Jordan Frame, from which we obtain a non-trivial f(R) and a non-trivial coupling between the fields \(\tilde{\phi }\) and \(\tilde{\psi }\) that yield parametric resonance in the Einstein Frame. This study is a first step towards building a mathematical relationship between the two frames that will allow further investigations on the production of primordial gravitational waves and black holes in f(R) theories.