A novel optical plastic fiber comprising a core region and having a center line along a longitudinal axis of the fiber is disclosed. The refractive index of the core region increases along a direction going from a periphery portion to the center line in any plane perpendicular to the center line, and the birefringence index of the core region varies along a direction going from the center line to a periphery portion perpendicular to the center line in any plane parallel to the center line and containing the center line.