Abstract Flow birefringence is examined in classical fluids near the critical point under oscillating incompressible flows. This effect originates from anisotropy in the fluctuation part of the dielectric tensor in the presence of a flow and is found to be enhanced near the critical point. When the flow is stationary, the birefringence phase shift δ is proportional to ξ 2 D in the weak shear regime and to D 1 3 in the strong shear regime where ξ is the correlation length and D is the shear rate. For the oscillatory case we examine only the weak shear regime. One characteristic feature is that δ becomes independent of T − T c when the frequency of the flow exceeds the average decay rate of critical fluctuations.