The absorption and photoluminescence spectrum of MWNTs have been measured. The optical frequency upconversion has been observed from MWNTs. The photoluminescence spectrum,which was observedonexcitationat1064nm,consistedofabroadstructuredband,peakingatapproximately780nm.Van Hove Singularities (VHS) in the density of states of multiwalled carbon nanotubes have been discoveredfromabsorptionspectrum.Thepeaks,correspondingtoVHS,locatedat685nm,719nm,894nm.The mechanism describing the underlying processes shows that the excitation involves double-photon absorption followed by non-radiative vibrational decay to the VHS states. Subsequently, the luminescence is produced by radiative transition.