The purpose of the present study was to examine the relationship among prior knowledge, cognitive load, flow, and achievement. Furthermore, an attempt was made to determine whether these variables could give meaningful influences to learning achievement. The participants of this study were 164 from Y Girls\' High School, and the study lasted five weeks beginning October to November in 2007. Prior knowledge, learning achievement, cognitive load and flow were measured by a set of questionnaires. The current study has reached the following conclusions: first, results of a correlation analysis has proven that a correlation exists among all the variables, particularly with the problem solving time and learning achievement, which showed a high correlation. Prior knowledge and learning achievement, and cognitive effort and achievement also show a relatively high correlation. Second, cognitive effort and the problem solving time, which are sub-variables of the cognitive load, and attention, interest, and curiosity, which are sub-variables of flow, are predicted by prior knowledge. The results of the study indicate that prior knowledge is a variable predicting cognitive load and flow. Third, it has been proven that prior knowledge, cognitive load, and flow predict achievement, particularly, prior knowledge, which is the highest predicting variable of achievement. Fourth, in order to determine whether cognitive load and flow act as mediating variables between prior knowledge and achievement, a hierarchical regression analysis was conducted. As a result, it has been shown that all the variables have mediating effects. Therefore, further research should focus on teaching strategies that can reduce the students\' cognitive load and improving their prior knowledge. In addition, after studying the relationship between flow and cognitive load, an explicit solution should be suggested to raise the level of study achievement.