Plants of Commelina benghalensis L. can produce four types of seeds, overground large(OL) seeds, overground small(OS) seeds, underground large(UL) seeds and underground small(US) seeds. Studies on these four types of plants showed results as follows: in contribution to new generation of individual plant, OL made the most while UL made the least, in reproductive allocation, plants from OL had most resource allocation to sexual reproduction yet UL had the least, which caused differences in reproductive effort and reproductive cost among four types of plants. Plants from UL had the least reproductive effort yet OL had the most. Reproductive costs of plants from large seeds were significantly less than those of plants from small seeds, and reproductive costs of plants from OS were also significantly less than those of plants from US, but differences between two types of large seeds were not significant.