In the last few years educational computer games have gained attention as a tool for facilitating learning in different sectors of society including but not limited to military, health, and education. However, advances in computer game technology continue to outpace research on its effectiveness. Few empirical studies have investigated the effects of educational games in the context of formal K-12 settings. The purpose of this study was to examine the effects of a series of mathematics computer games on mathematics achievement and motivation of high school students. In addition, the role of prior mathematics knowledge, computer skill, and English language skill of the participants on their mathematics achievement and motivation when they played the games were investigated. A total of 193 students and 10 teachers from an urban high school in the southeast of the United States of the America participated in this study. The teachers were randomly assigned to treatment and control groups. Students' mathematics achievement was measured using school district benchmark exams and a game performance test generated by the developers of the mathematics games. A mathematics motivation questionnaire based on Keller's (1987a) ARCS model of motivational design measured students' mathematics motivation. Multivariate Analysis of Co-Variance (MANCOVA) was conducted to analyze the data. In addition, interviews were conducted to cross validate the results of the quantitative data. The MANCOVA results indicated significant improvement of the mathematics achievement of the experimental versus control group. No significant improvement was found in the motivation of the experimental versus control group. However, a significant improvement was found on the motivation scores of the students who played the games in their school lab and classrooms compared to the ones who played the games only in the school labs. In addition, the findings indicated that prior mathematics knowledge, computer skill and English language skill did not play significant roles in achievement and motivation of the experimental group. Teachers' interviews revealed that these individual differences had indeed played significant roles in game-playing at the beginning of using the games, but the impacts gradually diminished as the students gained the required game-playing skills. The overall results indicated that the mathematics games used in this study were effective teaching and learning tools to improve the mathematics skills of the students. Using the games in mathematics education was suggested by the teachers as an appropriate alternative way of teaching, as one of the teachers stated: "This is definitely the way that we have to go to teach mathematics in the future." Mathematics games should be integrated with classroom activities if teachers want to increase mathematics class motivation. Teachers' helps and supports are vital in using the games effectively in a population with different prior mathematics knowledge, computer skills, and English language skills.