AbstractBased on the Mueller matrix approach, we have successfully designed and constructed a highly sensitive instrument for measuring optical birefringence (LB) by using a polarization modulation method and phase‐sensitive technique. Our instrument can measure the retardation of a sample up to 10−2 radian. Using this instrument, we have measured the residual birefringence of thermosetting resins, CR‐39 resin and epoxy resin, for opticaldisk substrates. CR‐39 resin results are summarized as follows: (1) Annealing is very effective in reducing LB of samples; (2) samples prepared from raw material 2 (monomer purity, 96%, oligomer, 4%) have smaller LB than those from raw material 1 (monomer purity, 99.9%); and (3) annealed samples prepared from 2 satisfy the requirement on LB to be used as optical‐disk substrates. For epoxy resin: (1) “Second curing” largely reduces LB of samples; (2) “slow cooling” is more effective in reducing LB of samples than “fast cooling;” (3) after the second curing, cutting does not induce any residual stress birefringence in samples; and (4) slow‐cooled samples have good optical properties to be used as optical disk substrates.