The invention provides process for preparing antibody-drug conjugates, preferably trastuzumab maytansinoid conjugate linked via non-cleavable linker comprising performing the conjugation reaction at lower temperature than ambient and/or at lower pH condition than neutrality. The process according to the present invention comprises conjugating the linker modified antibody with cytotoxic drug, preferably the SMCC-modified trastuzumab with a cytotoxic drug, preferably maytansinoid at a temperature lower than 20° C. and at a pH lower than 6.0, and thereby preparing an antibody-drug conjugate, preferably trastuzumab maytansinoid conjugate, T-DM1.
