In this research, the variation of spermatozoa traits among three male rainbow trout (Oncorhynchus mykiss) was used to examine how the relative influences of sperm density and duration of sperm motility account for sperm competition success. Sperm competition trial was conducted to study the fertilization ability of sperms from three male rainbow trout. The proportions of larvae sired by different males were quantified using DNA microsatellite analysis. Larvae sired by male number 2 dominated the offsprings this male sired 56% and 48% of the offspring checked in the paternity test using OMM1036 and Ocl8 loci respectively. Microsatellite DNA fingerprinting revealed that duration of sperm motility was conducive to sperm competition success. There was no significant relationship between fertilization success and either relative sperm count or duration of sperm motility sperm count showed an inverse relationship with competition success (P95%) when sufficient numbers (107) of spermatozoa per egg were used. Therefore, the fertilization success did not depend on whether sperm from one, or from a mixture of more males were used.