Understanding reproductive biology is important to elucidate the ecology and life history of animals and is needed to predict population dynamics and demography for conservation. The loggerhead turtle (Caretta caretta) is an endangered sea turtle species, and the vast majority of what is known about their reproductive biology has been learned from nesting females, with little information available on mature males and the time before reaching maturation. In order to increase the understanding about biology of maturation, size at maturity and puberty and the relationship between tail elongation and sexual maturation in loggerhead turtles in the North Pacific Ocean were investigated. The maturity stages of 54 males and 106 females, mostly captured at Kochi and Mie prefectures, Japan, were determined on the basis of gonadal development. There is no significant size difference between males and females in all maturity stages. The straight carapace length notch to tip (SCLn-t) at maturity was estimated as 82.1 cm. Tail elongation is the main secondary sexual characteristic of mature males. The elongation appears to begin when a male reaches to 65.8 cm in SCLn-t. The SCLn-t coincides with the size at puberty, which was estimated as 66.0 cm in this study.