The Heun function generalizes all well-known special functions such as Spheroidal Wave, Lame, Mathieu, and hypergeometric 2 F 1 , 1 F 1 and 0 F 1 functions. Heun functions are applicable to diverse areas such as theory of black holes, lattice systems in statistical mechanics, solution of the Schrödinger equation of quantum mechanics, and addition of three quantum spins.In this paper, applying three term recurrence formula [9], we consider asymptotic behaviors of Heun function and its integral formalism including all higher terms of A n 's. 1 We show how the power series expansion of Heun functions can be converted to closed-form integrals for all cases of infinite series and polynomial. One interesting observation resulting from the calculations is the fact that a 2 F 1 function recurs in each of sub-integral forms: the first sub-integral form contains zero term of A n s, the second one contains one term of A n 's, the third one contains two terms of A n 's, etc.Applying three term recurrence formula, we consider asymptotic behaviors of Heun functions and their radius of convergences. And we show why Poincaré-Perron theorem is not always applicable to the Heun equation.In the appendix, I apply the power series expansion and my integral formalism of Heun function to "The 192 solutions of the Heun equation" [34]. Due to space restriction final equations for all 192 Heun functions is not included in the paper, but feel free to contact me for the final solutions. Section 6 contains two additional examples using integral forms of Huen function.This paper is 4th out of 10 in series "Special functions and three term recurrence formula (3TRF)". See section 6 for all the papers in the series. The previous paper in series deals with the power series expansion in closed forms of Heun function. The next paper in the series describes the power series expansion of Mathieu function and its integral formalism analytically.