The morphology of conic markings was observed on the fracture surfaces of amorphous polysulfone, polyethersulfone and polyetherimide and semicrystalline polypropylene, polyphenylene sulfide and polybutylene terephthalate by using scanning electron microscopy. Most of conics exhibit the stepped stereo configuration, and the origin of secondary crack is depressed in the crack plane as a result of plastic deformation. The brittle-ductile transition of polymers can be analyzed by comparing the matching morphology of conic markings. According to computer simulations, the type I marking is determined by the ratio of the main to the secondary crack velocity while the type II marking is determined by the distances that two secondary cracks have covered before they meet. Variations of conic shapes can help to determine the changes of crack growth velocity.