The reproductive biology of pen shell, Pinna Pectinata , in the northern offsea of Shandong were studied based on 851 samples caught from 1994 to 1998. The results showed that: The ovarian condition is divided into 6 maturation stages according to the morphologic character of ovary, gonadosomot ic index, ova diameter, and ova structure. The gonad muturates once a year and can be divided into 4 phases, that is evolution, proliferation, muturation, and rest . The spawning period is from June to August. and the peak period is from the f irst ten days of July to middle of August. The mutured age is one year. The sex radio is 53. 50..46. 50. The individual absolute fecundity is ranged from 1731. 0 to 53815. 9 thousand. The fecundity per gonad weight is 2844. 1 thousand/ g in average.