The determination of the CP-violating phase $\phi_s$ in $B^0_s \to J/\psi\phi$ decays through a time dependent angular analysis is one of the key goals of the LHCb experiment. Here, the first LHCb measurement of the mixing phase $\phi$s using a data sample corresponding to 36 pb$^{−1}$ of proton-proton collision taken in 2010 at a center-of-mass energy of 7 GeV is presented. The analysis of (860 $\pm$ 60) selected $B^0_s \to J/\psi\phi$ candidates constrains $\phi_s$ to $−2.7 < \phi_s < −0.5$ at 68% CL.