This study aimed to assess the effects of fifth-graders’ mathematics self-efficacy (MSE) on their mathematical achievement (MA) in school, as well as examine the effects of gender, family socio-economic status (SES), and parenting styles on MSE. A student’s background sheet and a mathematics self-efficacy instrument were administered to 1,244 fifth-graders for gathering data, along with their mathematical achievement scores in school. Corresponding statistical analyses were applied to the obtained data. The findings showed that fifth-graders’ family SES and parenting styles were ascertained as critical elements in the development of their mathematics self-efficacy, while gender was not significantly influential but developmentally noticeable. It also revealed that their MSE ratings could effectively predict their MA. Consequently, suggestions derived from findings and discussions were proposed for further improvement of these fifth-graders’ MSE and the future study.