The effects of various annealing temperatures and durations on the structure and physical properties of polypropylene (PP) slit-film yarns were investigated. Annealing led to a more developed crystal structure, even when the yarns were drawn before heat treatment. Annealing at $140^{circ}C$ led to better developed crystal structures than that at $100^{circ}C$. Annealing primarily affected the crystalline structure, whereas little change was observed in the molecular orientation of the yarns. The slitfilm yarns annealed at $140^{circ}C$ showed less than 1% shrinkage. A very low-shrinkage slit-film yarn was obtained on annealing for 10 min at $140^{circ}C$ or 20 min at $100^{circ}C$.