This study aims at identifying the effectiveness of the Constructive Model of Trowbridge and Bybee in Teaching Mathematics on Students Achievement and their algebraic Thinking of Basic Education grade nine Students through comparing this model with the common traditional teaching approach. The study population was composed of basic education grade nine students in public schools of Muscat governorate of the 2010/2011 school year. The sample included 69 students from two classrooms in Kaa'b bin Zaid Basic Education School. The sample was divided into two groups. The first group was experimental of 34 students who were taught mathematics through the constructive model of Trowbridge and Bybee, meanwhile the second group of 35 students acted as the control group and they were taught by using the traditional methods of teaching, Implementation of the study lasted for 30 school periods. The researcher used in this study two tools. The first one was algebraic Achievement test and the second was algebraic thinking test. The researcher developed an algebraic Achievement test to assess the extent to which students of the sample acquire concepts and skills included in the equations and functions unit in the prescribed mathematics textbook for grade nine. On the other hand, he developed an algebraic thinking test making use previous studies to assess levels of students with regard to algebraic thinking skills such as using symbols, algebraic relations, using multiple presentation and using patterns and generalizations. The researcher applied both tests on an exploratory group to confirm the reliability of the tests. The Cronbach Alpha coefficient of reliability was 0.86 for the algebraic Achievement test and 0.83 for the algebraic thinking test. The results of the study shows that there is a statistical significant difference at the level of a = 0.05 between the two means of performance of the experimental group and the control group with regard to the equations and functions unit. The difference was in favor of the experimental group. On the basis of the results of this study, the researcher recommends that special activities should be included in mathematics courses to develop algebraic thinking among students. He also recommends similar studies and researches should be carried out at different school levels using other variables.