Phytoseiulus persimilis is an important biological control agent, commercially produced worldwide.  To increase understandings of its reproduction, herein we provided quantitative descriptions of the inflation and deflation of its spermathecae as a function of time and copulation duration, and the quantitative impact of copulation termination on volume of spermatheca vesicle, fecundity, and offspring sex ratio.  After mating started, at least one spermatheca started to inflate immediately, but no egg was produced until spermatheca volume reached 1 021 μm3.  Beyond this size, cumulative fecundity was linearly correlated with vesicle volume.  Producing one egg required 36 μm3 volume increase of the vesicle.  Each spermatheca vesicle reached its peak size at the end of mating duration (ca. 2.38 h after mating started), and started to shrink immediately.  In 24 h, ca. 71% individuals had one completely shriveled spermatheca, while in 72 h all individuals had at least one shriveled spermatheca.  The estimated maximum cumulative fecundity per female after a single mating is (69.4±7.7) eggs.  No significant impact of mating termination on offspring sex ratio was observed after mating duration reached 60 min, while higher proportion of male offspring was observed when mating duration is 15 or 30 min.  This study is the first step to investigate possible male impact on offspring sex ratio in P. persimilis.  Our results suggested the number of sperms received during mating be a restriction factor of P. persimilis offspring sex ratio regulation.