This study examined the impact of different levels of task difficulty and expertise on self-efficacy judgments. In addition, the study examines how self-efficacy judgments affect the amount of mental effort investment and task performance under different levels of task difficulty and expertise. Results from this study are used to build a performance model that helps illustrate the relationship among these variables. A quantitative experimental design was used for the study. A strategic computer game, WarCraft III, was used to examine the subjects' levels of expertise and task performances. Three tasks with different difficulty levels--Easy, Moderate, High were selected for the experiment. A developed program that can randomly present a pop-out window on the computer screen was used as a secondary task to interrupt subjects' engagement of the primary task and to examine subjects' amount of mental effort investment. In addition, questionnaires were used to examine the subjects' perceived self-efficacy and self-reported mental effort. Sixty-six subjects participated in the study. One-way ANOVA result showed that there was no significant difference between easy and moderate tasks. As a result, data from these two tasks were combined as "Normal" task. Data of "Normal" and "High" tasks were then used to test fourteen hypotheses. Results from t-test showed that there was a significant and positive relationship between perceived self-efficacy and expertise. Subjects with high level of expertise have high self-efficacy and subjects with low level of expertise have low self-efficacy. However, levels of task difficulty and perceived self-efficacy were negatively correlated. The higher the levels of task difficulty are, the lower the perceived self-efficacy is found. Results from the correlation and regression analysis also indicated that the relationship between perceived self-efficacy and self-reported mental effort was negative. However, the scatter plot suggested that the relationship between perceived self-efficacy and dual-task measured mental effort roughly revealed an inverted-U shape--as efficacy increased, effort also increased until tasks became challenging and then effort appeared to decrease. Analysis also supported the conclusion that there was a positive relationship between perceived self-efficacy and task performance. The relationship between levels of task difficulty and secondary mental effort measures was also positive and the relationship between levels of task difficulty and task performance was negative. Moreover, the results also indicated that the relationship between expertise and mental effort was negative while the relationship between expertise and task performance was positive. Based on these results, a performance model was established and path analysis was used to examine the model. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]