Deformation and breakup mechanisms of an immiscible drop were investigated experimentally in a screw channel flow, which mimics the flow in the metering section of the single screw extruder It was observed that the deformation/breakup mechanisms are far distinct from the well-known routes for the linear flow, mainly because of the complex flow history that can be characterized by the repetition of strong/weak shear the continuously varying shear direction and the acceleration/deceleration. The experimental observations include: comet like thread formation with the large bulbous head and the long narrow tail; the initiation of the breakup at the entrance of the low-shear zone; breakup by asymmetric end pinching; breakup by the growth of the capillary wave, propagating from the tail toward the head, monotonically decreasing droplet size distribution from the head to tail. The mechanism for the comet-like thread formation has been investigated by using the numerically obtained flow field.