A method for priming an extracorporeal blood circuit of an extracorporeal blood treatment apparatus, before starting patient treatment, the extracorporeal blood treatment apparatus, comprising: a blood treatment device having a first blood chamber and a second fluid chamber separated from one another by a semipermeable membrane; a blood warmer; the extracorporeal blood circuit comprising a blood withdrawal line and a blood return line coupled to the extracorporeal blood treatment device, wherein the blood return line presents a heating zone coupled to the blood warmer; a blood pump configured to be coupled to a pump section either of the blood withdrawal line or of the blood return line; at least a post-infusion line (13, 13') connected to the blood return line upstream of the heating zone; an air trapping device placed on the blood return line upstream of the heating zone; wherein the priming method comprises the step of: - filling the extracorporeal blood circuit (6, 7) of the extracorporeal blood treatment apparatus with a priming fluid and making the priming fluid flowing at least through the extracorporeal blood treatment device and through at least the blood return line towards the heating zone of said blood return line; wherein, during an initial time interval (ΔT1), the priming fluid is diverted into a secondary post-infusion line, in fluid communication with the blood return line upstream of the heating zone and connected to the blood return line downstream of the heating zone, to bypass the blood warmer operatively active on said heating zone; wherein, after said initial time interval, the diversion is interrupted so that the priming fluid flows through the air trapping device placed on the blood return line upstream of the heating zone and then through said heating zone towards a terminal end of the blood return line. a blood treatment device having a first blood chamber and a second fluid chamber separated from one another by a semipermeable membrane; a blood warmer; the extracorporeal blood circuit comprising a blood withdrawal line and a blood return line coupled to the extracorporeal blood treatment device, wherein the blood return line presents a heating zone coupled to the blood warmer; a blood pump configured to be coupled to a pump section either of the blood withdrawal line or of the blood return line; at least a post-infusion line (13, 13') connected to the blood return line upstream of the heating zone; an air trapping device placed on the blood return line upstream of the heating zone; - filling the extracorporeal blood circuit (6, 7) of the extracorporeal blood treatment apparatus with a priming fluid and making the priming fluid flowing at least through the extracorporeal blood treatment device and through at least the blood return line towards the heating zone of said blood return line; wherein, during an initial time interval (ΔT1), the priming fluid is diverted into a secondary post-infusion line, in fluid communication with the blood return line upstream of the heating zone and connected to the blood return line downstream of the heating zone, to bypass the blood warmer operatively active on said heating zone; wherein, after said initial time interval, the diversion is interrupted so that the priming fluid flows through the air trapping device placed on the blood return line upstream of the heating zone and then through said heating zone towards a terminal end of the blood return line.