This study aimed at investigating the effectiveness of guided inquiry strategy in developing grade eight students' achievement and mathematical reasoning. The sample consisted of (62) grade eight female students and was divided into two groups: An experimental group of (31) students and a control group which consisted of (31) students. Moreover, the students of both groups were divided into dependent and independent cognitive style students. The experimental group consisted of (16) independent students and (15) dependent students using the Group Embedded Figures Test (GEFT). The control group consisted of (15) independent students and (16) dependent students. The two groups were checked before the experiment to make sure that they were equivalent in terms of students' achievement and their mathematical reasoning. After that the experimental group was taught using the guided inquiry strategy, and the control group taught according to the traditional method. In order to measure the effect of the suggested teaching strategy on the experimental group, the researcher used three tests. First, the Group Embedded Figures Test (GEFT) prepared by Witken and translated by AlSharqawi and Al- Shaikh. Second, an achievement test and mathematical reasoning test which are prepared by the researcher. Thirdly, three different experimental designs were used because of the differences in the variables of the study. To answer the questions of this study and discuss the hypotheses, the researcher analyzed the data with the SPSS program. This study reached the following results: 1. The students, in the experimental group, who were taught with guided inquiry strategy surpassed the students in the control group who were taught with the traditional method in the achievement test. 2. The dependent students in the experimental group surpassed their counterparts in the control group in the achievement test. 3. There was no significant difference in mean performance between independent and dependent students using the achievement test. 4. There was no significant difference, in the mathematical reasoning test, between the mean of students' scores in the experimental group and the mean of the students' performance in the control group. Moreover, the results of the study showed that there was no significant difference, in the mathematical reasoning test, between the means scores of the independent students in the experimental group and the means scores of the independent students in the control. Furthermore, the results of the study revealed that there is no significant difference, in the mathematical reasoning test, between the means scores of the independent students in the experimental group and the means scores of the independent students in the control group. In the light of these results, the researcher recommended that teachers should use the guided inquiry strategy in mathematical lessons as it has an important role in enhancing students' achievement especially dependent students. Moreover, the researcher recommended that teachers should consider the questioning inquiry side when teaching mathematics.