This study aimed to identify common errors in algebra among the eighth grade students, and determine the impact of a proposed training program on helping mathematics teachers acquire the skills to treat these errors. The researcher used the descriptive and quasi experimental methodologies. The sample of the descriptive study consisted of (445) male and female students. Based on the educational literature, the content analysis and an open questionnaire oriented to teachers of math, the researcher designed a diagnostic test to identify the common errors in algebra among eighth grade students. The diagnostic test was applied after the checking of its validity and its reliability. The selected sample for the quasi experimental study consisted of (9) teachers of mathematics teachers in Muscat who were administered a training program prepared by the researcher. To measure the degree of acquisition of skills was used the pretest and posttest. To determine the impact of the training program was used (T-test) for one sample. The study found the following results: 1-The presence there is a statistically significant difference between the average performance of mathematics teachers on the performance for treatment common errors skills before and after the training program for the benefit in favour of the post application. 2- The existence of a large number of common errors on basic algebraic concepts, made by students in the eighth grade, can return caused by confusion between the concepts, the use of wrong generalizations of algebraic, and the teaching of algebra generalization of the arithmetic, without recognizing the relationship between algebra and arithmetic, specifically the points of agreement and disagreement between them. In light of the results of the study was to provide a number of recommendations adopt a general strategy to provide on-job in service training for the teachers employing the modern teaching methodologies such the Diagnostic – Prescriptive Teaching, conduct diagnostic tests prepared by the Ministry of Education to identify common errors in mathematics in order to take these errors into account when preparing curricula. This study proposes conducting similar studies to this at various educational levels on a large sample of teachers and students in order to disseminate the results of this search.