This paper presents 2 new classes of the Bessel functions on a compact domain[0, T] as generalized-tempered Bessel functions of the first-and second-kind which are denoted by GTBFs-1 and GTBFs-2. Two special cases corresponding to the GTBFs-1 and GTBFs-2 are considered. We first prove that these functions are as the solutions of 2 linear differential operators and then show that these operators are self-adjoint on suitable domains. Some interesting properties of these sets of functions such as orthogonality, completeness, fractional derivatives and integrals, recursive relations, asymptotic formulas, and so on are proved in detail. Finally, these functions are performed to approximate some functions and also to solve 3 practical differential equations of fractionalorders. KEYWORDS Bagley-Torvik equation, Bessel functions, fractional derivatives and integrals, Gaussian quadrature rules, relaxation-oscilation equation, self-adjoint operator, spectral Galerkin method, tempered fractional derivatives and integrals Math Meth Appl Sci. 2017;40:6389-6410.wileyonlinelibrary.com/journal/mma Esmaeili and Shamsi 30 were the first authors who introduced a new class of the Jacobi polynomials to solve some fractional differential equations. It is worthwhile noting that the new set, generally due to the nonpolynomial nature, is in fact, a set of orthogonal basis functions. These basis functions are also performed to solve some fractional optimal control problems and calculus of variations. [31][32][33] Later, Zayernouri and Karniadakis 34 introduced 2 new classes of Jacobi functions, which are orthogonal with respect to some suitable weight functions, as the eigenfunctions of some fractional Sturm-Liouville eigenvalue problems on [−1, 1]. They also used these basis functions to solve various fractional differential equations. (See for instance Zayernouri and Karniadakis 35-37 and other papers of the same authors. Also see Chen et al 38 and references therein.) After them, Khosravian-Arab et al 39 introduced 2 new classes of generalized Laguerre functions, which are also orthogonal with respect to some weight functions on [0, ∞).As we are aware, one of the most interesting and very practical functions which arises frequently in mathematical physics is the Bessel function(s) (BF(s)). From the historical point of view, the BFs arose in Daniel Bernoulli's investigation of the oscillations of a hanging chain, Euler's theory of the vibration of a circular membrane, and Bessel's studies of planetary motion. Recently, several applications of BFs have been discovered in physics and engineering such as propagation of waves, elasticity, fluid motion, and in many problems of potential theory and diffusion involving cylindrical symmetry. [40][41][42] Although, the BFs are usually known as the solutions of a second-order differential equation, so-called as Bessel's differential equation, which arises by the use of the separation of the variables for various problems in mathematical physics, but, in fact, there is a very close connection betw...