In this paper we investigate the existence of nontrivial solutions for the following fractional boundary value problem: $$\begin{aligned} \left\{ \begin{array}{ll} \dfrac{d}{dt}\Bigl (\dfrac{1}{2} {_0D_t^{-\beta }(u'(t))}+\dfrac{1}{2} {_tD_T^{-\beta }(u'(t))}\Bigr )+\nabla F(t,u)=0,\quad a.e.\,\, t\in [0,T], \\ u(0)=u(T)=0, \end{array} \right. \end{aligned}$$ ddt(120Dt-β(u′(t))+12tDT-β(u′(t)))+∇F(t,u)=0,a.e.t∈[0,T],u(0)=u(T)=0, where $$_0D_t^{-\beta }$$ 0Dt-β and $$_tD_T^{-\beta }$$ tDT-β are the left and right Riemann-Liouville fractional integrals of order $$0\le \beta <1$$ 0≤β<1 , respectively, and $$\nabla F(t,u)$$ ∇F(t,u) is the gradient of F(t, u) at u. The novelty of this paper is that, when the nonlinearity F(t, u) involves a combination of superquadratic and subquadratic terms, we present some reasonable assumptions and establish one new criterion to guarantee the existence of at least two nontrivial solutions. Recent results in the literature are generalized and significantly improved.