A process for continuously preparing a polyolefin composition comprising polyolefin and carbon black in an extruder device preparing polyolefin pellets of the polyolefin composition, the process comprising the steps (i) supplying polyolefin in form of a polyolefin powder at a flow rate to a mixing device; (ii) measuring the flow rate of the polyolefin powder supplied to the mixing device or measuring a flow rate of the polyolefin pellets prepared in the extruder device; (iii) supplying carbon black to the mixing device; (iv) optionally supplying one or more further additives to the mixing device; (v) when measuring the flow rate of the polyolefin powder supplied to the mixing device, adjusting the flow rates of the carbon black and the optionally supplied one or more further additives to the mixing device in response to the measured flow rate of the polyolefin powder or, when measuring the flow rate of the polyolefin pellets prepared in the extruder device, adjusting the flow rate of the polyolefin powder to the mixing device in response to the measured flow rate of the polyolefin pellets and either keeping the flow rates of the carbon black and the optionally supplied one or more further additives supplied to the mixing device constant or also adjusting the flow rates of the carbon black and the optionally supplied one or more further additives supplied to the mixing device in response to the measured flow rate of the polyolefin pellets; (vi) mixing the polyolefin powder, the carbon black and the optionally supplied one or more further additives in the mixing device to form a powder mixture, (vii) transferring the powder mixture obtained in step (vi) from the mixing device into the extruder device; (viii) heating the powder mixture to form a melt and homogenizing the melt within the extruder device to form a molten polyolefin composition; and (ix) pelletizing the molten polyolefin composition, thereby yielding the polyolefin pellets. (i) supplying polyolefin in form of a polyolefin powder at a flow rate to a mixing device; (ii) measuring the flow rate of the polyolefin powder supplied to the mixing device or measuring a flow rate of the polyolefin pellets prepared in the extruder device; (iii) supplying carbon black to the mixing device; (iv) optionally supplying one or more further additives to the mixing device; (v) when measuring the flow rate of the polyolefin powder supplied to the mixing device, adjusting the flow rates of the carbon black and the optionally supplied one or more further additives to the mixing device in response to the measured flow rate of the polyolefin powder or, when measuring the flow rate of the polyolefin pellets prepared in the extruder device, adjusting the flow rate of the polyolefin powder to the mixing device in response to the measured flow rate of the polyolefin pellets and either keeping the flow rates of the carbon black and the optionally supplied one or more further additives supplied to the mixing device constant or also adjusting the flow rates of the carbon black and the optionally supplied one or more further additives supplied to the mixing device in response to the measured flow rate of the polyolefin pellets; (vi) mixing the polyolefin powder, the carbon black and the optionally supplied one or more further additives in the mixing device to form a powder mixture, (vii) transferring the powder mixture obtained in step (vi) from the mixing device into the extruder device; (viii) heating the powder mixture to form a melt and homogenizing the melt within the extruder device to form a molten polyolefin composition; and (ix) pelletizing the molten polyolefin composition, thereby yielding the polyolefin pellets.