The processes of energy migration in upconvertion nanocrystals (UCNPs) governing the quantum efficiency un-der pulse excitation at 975 nm, which is a decisive factor for the widespread use of UCNPs, have been studied. Thetreatment by picosecond laser radiation leads to a controlled nanotransformation of a three-dimensional lumines-cent structure into a one-dimensional one through the formation of particles with a structure resembling a “medu-sa”. The upconversion process in the one-dimensional structure occurs due to the energy migration between Yb3+,as in the case of nanoparticles. An approach is proposed for evaluating the efficiency of nonradiative energy transferin a complex of UCNPs with a fluorophore. It takes into account the contribution of energy migration between sen-sitizer ions. The use of UCNPs in photothermal therapy is shown to be promising due to the large absorption crosssection of the Yb3+ sensitizer. The cellular response to hyperthermia involving UCNPs is demonstrated by measuringheat shock protein expression.