Marked shrinkage behavior when heated is typical of semicrystalline polymer fibers such as polypropylene (PP) and polyacrylonitrile (PAN). Shrinkage of PP and PAN fibers may give the possibility to control the spalling tendency of fiber concrete under the heat exposure of fire. Cut staple fibers are normally delivered for concrete reinforcement. Modern methods for continuous fibers cannot be used by the end-user for shrinkage determination of commercial staple fiber grades. The shrinkage of five different commercial staple fibers specially designed for concrete reinforcement was studied under a hot-stage microscope. Significant differences in cumulative shrinkages of the various PP and PAN fibers were detected, shrinkages being 3-15% with PP fibers and 6-7% with PAN fibers at a temperature of 150-170 degrees C. At about 160-165 degrees C, PP fibers melt, whereas PAN fibers continue shrinking. Hot-stage microscopy provides a simple and a relatively accurate method for estimating thermal shrinkage of staple PP and PAN fibers, the deviations from measured average values remaining typically at 10-15%.