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Ba₃CoSb₂O₉ is one of few prototypical materials for the two-dimensional equilateral triangular lattice antiferromagnet. We present a systematic study on effect of quenched disorder on magnetic properties in the Sr doped single-crystalline sample of Ba_2.8Sr_0.2CoSb₂O₉ by means of DC and AC susceptibilities, specific heat and advanced neutron scattering. DC susceptibility in Ba_2.8Sr_0.2CoSb₂O₉ confirms an antiferromagnetic ordering below 2.8 K with an easy-plane anisotropy. AC susceptibility as a function of magnetic field measured at 0.02 K indicates a phase transition around 8 T. Evolution of the transition temperature as a function of field was determined by specific heat measurements in Ba_2.8Sr_0.2CoSb₂O₉. We will also discuss effect of side disorder on the magnetic structure in Ba_2.8Sr_0.2CoSb₂O₉ using the single-crystal neutron diffraction technique.