Summary: We consider the root function expansion of the nonlocal Strum-Liouville problem \[ y''-q(t)y=\lambda y,\;\alpha_0y(0)+ \beta_0 y'(0)=0,\;\chi(y)=\varphi (y')+\psi(y)=0, \] where \(q\in L^1[0,a]\) and \(\varphi,\psi\) are continuous linear functional in \(C[0,a]\). By certain assumptions imposed on this functional, theorems for completeness and basis property of the root functions of this problem are proved.