A blood sampler includes a hollow transparent barrel including a reduced collection portion located at a front end thereof and terminating in a reduced coupling portion, an outlet defined in the coupling portion and a plurality of ribs spaced around and raised from an inside wall near an opposing rear end thereof, a piston axially movably fitted into the hollow transparent barrel and having a screw hole and peripheral ribs, a cap detachably capped onto the collection portion to seal the outlet, and a plunger having a screw rod located at a front end thereof and threaded into the screw hole of the piston for allowing movement the piston with the plunger. The hollow transparent barrel enables collecting blood sample from a patient, separating the collected blood sample into platelet-rich plasma, platelet-rich plasma and serum in a centrifuge and injecting the collected platelet-rich plasma into a patient body.