To determine the relative efficiency and mesh selectivity of gill net for the Pacific saury, Cololabis saira, a series of fishing experiments was carried out in the Northwest Pacific Ocean from August 13 to October 13 in 2002, using gill nets of different mesh size(30, 33, 35, 37, 39 and 42mm) constructed from two kinds of twine material(monofilament, twisted multifilament nylon web). The relative efficiency of two material gears was expressed as the ratio obtained by dividing monofilament catch by multifilament catch in number. The master selection curve of each material gear was estimated by applying the extended Kitahara's method. The catch of experimental gears is mostly Pacific saury(98.6%), Cololabis saira. The kinds of bycatch are common squid(0.7%), Pacific mackerel(0.6%), etc. Catch comparisons in the two gears showed that monofilament nylon nets are 1.7 times more efficient. The optimum values in monofilament and multifilament gill net for Pacific saury are 8.28 and 8.23, respectively.