The major purpose of this study was to investigate whether successful practice trials in throwing would mediate the relationship between perceived throwing competence and skill improvement in throwing among 1 st graders in physical education. A correlational research design was utilized to examine the mediating relationships among variables. Participants (n=33) were from three 1st grade classes and received 120 minutes of throwing instruction and practice during the 4-day throwing unit. Participants' perceived throwing competence was assessed using a self-reported survey before and after the unit. Their throwing performance was assessed using the Test of Gross Motor Development-2 (TGMD-2) and Component of Overhand Throw before and after the unit. A sub-sample of 18 participants was selected and their total number of practice trials and percentage of successful practice trials during practice and games were lived coded during the unit. Descriptive statistics (i.e., means and frequencies) and regression analyses, and paired sample t-test were used to analyze data. The results indicated that successful practice trials in throwing did not mediate the relationship between perceived throwing competence and skill improvement in throwing as measured by the TGMD-2, but completely mediated the relationship between perceived throwing competence and skill improvement in throwing as measured by Component Sequence. Furthermore, participants significantly improved their throwing performances and perceived throwing competence from pretest to posttest. The findings of the study suggested that physical education teachers should employ motivational strategies to increase students' efficacy beliefs and focus their instruction on increasing their percentage of successful practice trials in order to improve their skill proficiency. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]