Predatory lady beetles (Coleoptera: Coccinellidae) have been introduced to North America in recent decades, raising concerns of adverse impacts on native species, especially lady beetles. In northern Utah, Coccinella septempunctata (Linnaeus) first appeared in 1991, and now is predominant among predatory ladybirds in alfalfa fields. For insects in general, fecundity is positively correlated with body size of females. However, the relationship between the body size/fecundity and the invasion success is not well understood. We tested the hypothesis that an invasive species has a larger body size and higher fecundity than any other sympatric native species, which might explain their successful invasion. Native lady beetles, C. transversoguttata richardsoni (Brown), Hippodamia convergens Guerin, H. quinquesignata (Kirby), and H. sinuate crotchi Casey, and an invasive lady beetle, C. septempunctata were collected from the alfalfa fields in spring 2007. We measured egg production, egg volume, egg fertility and number of ovarioles as fecundity parameters. The correlations between body size and all measures of fecundity were analyzed and compared between species. Our results show that the invasive C. septempunctata has the largest body size among five species, more ovarioles, and produce relatively small eggs but large numbers of eggs. Such high fecundity of C. septempunctata related to the body size might explain their successful invasion in this habitat.