We present a new set of radial-velocity measurements of the spectroscopic binary star HD 165052 obtained by disentangling of high-resolution optical spectra. The longitude of the periastron ({varpi}=60+/-2{deg}) shows a variation with respect to previous studies. We have determined the apsidal motion rate of the system {dot{varpi}}=12.1+/-0.3{deg}/yr, which was used to calculate the absolute masses of the binary components: M_1_=22.5+/-1.0M_{sun}_ and M_2_=20.5+/-0.9M_{sun}_. Analysing the separated spectra we have re-classified the components as O7Vz and O7.5Vz stars.