In this paper , we present both timing and spectral analysis of the outburst of 4U 0115+63 in April – May 2008 with INTEGRAL and RXTE observations . We have determined the spin period of the neutron star at \sim 3.61430 \pm 0.00003 s , and a spin up rate during the outburst of \dot { P } = ( -7.24 \pm 0.03 ) \times 10 ^ { -6 } { s~ { } d ^ { -1 } } , the angle of periapsis \omega = 48.67 ^ { \circ } \pm 0.04 ^ { \circ } in 2008 and its variation ( apsidal motion ) \dot { \omega } = 0.048 ^ { \circ } \pm 0.003 ^ { \circ } { yr } ^ { -1 } when comparing with previous measured values of \omega . We also confirm the relation of spin-up torque versus luminosity in this source during the giant outburst . The hard X-ray spectral properties of 4U 0115+63 during the outburst are studied with INTEGRAL and RXTE . Four cyclotron absorption lines ( fundamental at \sim 10 - 15 keV , 2nd harmonic at \sim 22 keV , 3rd at \sim 34 keV and 4th at \sim 45 keV ) are detected using the spectra from combined data of IBIS and JEM-X aboard INTEGRAL in the energy range of 3 – 100 keV . Two cyclotron absorption lines are measured from the spectra combining PCA and HEXTE data in the energy band of 4 – 50 keV . The 5 – 50 keV luminosities at an assumed distance of 7 kpc are determined to be in the range of ( 1.5 - 12 ) \times 10 ^ { 37 } { ergs~ { } s ^ { -1 } } during the outburst . The fundamental absorption line energy varies during the outburst : around 15 keV during the rising phase , and transiting to \sim 10 keV during the peak of the outburst , and further coming back to \sim 15 keV during the decreasing phase . The variations of photon index show the correlation with the fundamental line energy changes : the source becomes harder around the peak of the outburst and softer in both rising and decreasing phases . This correlation and transition processes during the outburst need further studies in both observations and theoretical work . The known relation of the fundamental line energy and X-ray luminosity is confirmed by our results , however , our discoveries suggest that some other factors besides luminosity play the important role in fundamental line energy variations and spectral transitions .