The highly efficient photoluminescence (PL) from diamond nanoparticles of different size (diameter) D 5, 100, 400, 700, and 1000 nm was observed. The PL was induced by laser pulsed ultraviolet (UV) light [lambda(exc) = 354.7 nm (3.48 eV), tau = 10 ns] as well as by the infrared (IR) one [lambda(exc) = 1064 nm(1.16 eV), tau = 10 ns]. The laser light intensity dependences of the PL yield reveal multiphoton (MP) processes of the PL excitation. The PL is a result of a two-photon absorption (total energy 6.96 eV) in the case of larger size particles (100-1000 nm) excited by the UV light, but for 5 nm particles three photons (the total energy 10.64 eV) need to be provided. A five-photon absorption process occurs in the case of the IR light (total energy 5.8 eV). It is assumed that the PL results from structural defects, impurities, and the graphite-like phase excited by a relaxation of the MP-produced excitons (the band gap of diamond E-g = 5.5 eV). With the UV one-photon excitation, the intrinsic PL from the mentioned species with the same bands was observed, supporting the interpretation. The dynamics of MP excitation has been analyzed with respect to the size of particles. The amorphization and rearrangement of sp(3) sites of the diamond lattice into sp(2) sites of graphite due to MP absorption is assumed to be one of three stages of exciton energy relaxation and was supported by the Raman spectroscopy and TEM images. The typical Swan bands of electronically excited C-2 species dominate the spectra at the UV and IR light intensity above the threshold for vaporization, It was concluded that the MP-excited PL precedes the emission accompanying the laser vaporization process.