  In this paper, we study the following fractional Schr\"odinger equation: \[
\left\{\begin{gathered}
  {(- \Delta)^s}u + mu = f(u){\text{in}}{\mathbb{R}^N}, \hfill
  u \in {H^s}({\mathbb{R}^N}),{\text{}}u > 0{\text{on}}{\mathbb{R}^N}, \hfill
\\ \end{gathered} \right. \] where $m>0$, $N>2s$, ${(- \Delta)^s}$, $s \in
(0,1)$ is the fractional Laplacian. Using minimax arguments, we obtain a
positive ground state solution under general conditions on $f$ which we believe
to be almost optimal.
