A method of preparing a prefilled syringe (1), comprises the steps of: (i) obtaining a syringe barrel (11) having an open end (111) and tip (112) with an orifice (113) essentially opposite to the open end (111), and a needle adaptor cap (12) assembled on the tip (112) of the syringe barrel (11), wherein the needle adaptor cap (12) has a rubber element (121) tightly sealing the orifice (113) of the tip (112) of the syringe barrel (11) and the syringe barrel (11) together with the needle adaptor cap (12) assembled on the tip (112) of the syringe barrel (11) is sterilized by a first sterilizing comprising a main step of exposing the syringe barrel (11) together with the needle adaptor cap (12) assembled on the tip (112) of the syringe barrel (11) to ethylene oxide for about 5 hours to about 50 6ours at a relative humidity of about 40% to about 100% and at a temperature of about 30°C to about 60°C； (ii) filling a drug substance through the open end (111) of the syringe barrel (11) or through the orifice (113) of the syringe barrel (11) into an interior of the syringe barrel (11)； (iii) sealing the interior of the syringe barrel (11) by advancing a rubber stopper (13) through the open end (111) of the syringe barrel (11)； packaging the syringe barrel (11) together with the rubber stopper (13) sealing the interior of the syringe barrel (11) and the needle adaptor cap (12) assembled on the tip (112) of the syringe barrel (11)； and (iv) second external surface sterilizing the packaged syringe barrel (11) together with the rubber stopper (13) sealing the interior of the syringe barrel (11) and the needle adaptor cap (12) assembled on the tip (112) of the syringe barrel (11). The second external surface sterilizing comprises a main step of exposing the packaged syringe barrel (11) together with the rubber stopper (13) sealing the interior of the syringe barrel (11) and the needle adaptor cap (12) assembled on the tip (112) of the syringe barrel (11) to at least 6 pulses of va
