  LS I +61$^\circ$303 is a rare representative of the gamma-ray binaries with a
compact object known to be a pulsar. We report on the periodicity and spectral
analysis of this source performed with more than 14 years of Fermi/LAT data.
The periodicity of LS I +61$^\circ$303 is strongly energy dependent. Two
periods $P_1 = 26.932\pm 0.004 (stat)\pm 0.008 (syst)$ and $P_2 = 26.485 \pm
0.004 (stat)\pm 0.007 (syst)$ are detected only at $E>1$ GeV and at $E<0.3$ GeV
correspondingly. Within $1\sigma$ (stat+syst) the periods are consistent with
orbital ($P_2$) and beat orbital/superorbital ($P_1$) periods. We present the
orbital light curves of the system in several energy bands and the results of
the spectral analysis. We discuss the possible origin of the change in the
variability pattern between 0.1 and 1 GeV energy.
