The study investigates the comparative effectiveness of teaching English grammar by using deductive and inductive teaching models. The study also attempts to see which of these two methods has a positive effect on the grammar academic achievement of the university students and elementary school students in Jordan so it answers the following questions: (1) What is the effect of inductive method on grammar achievement compared with deductive method at university level?; and (2) What is the effect of inductive method on grammar achievement compared with deductive method at elementary stage? To answer the questions of the study, the researcher prepares two programs based on inductive and deductive methods for each level based on its syllabus. The sample consists 180 students; eighty at the 1st year level in English departments at university level and one hundred at the elementary stage. The participants of the study consisted of four assigned sections. Firstly, at the university level, two colleges are randomly selected out of the eighteen faculties of Albalqa' Applied University; two sections are randomly selected; one group is randomly assigned as the 1st experimental group (by using inductive method) and the second experimental group (by using deductive method). Similar process is adopted in the case of school students at the elementary stage. The relevant pre-tests are administered to the students of both groups at each stage (university and school) to make sure that the groups are equivalent at the time of starting the experiment. The researcher designs two grammar achievement tests as the instruments of this study (one for the elementary stage and one for university level). The instrument of each stage consists of two achievement tests (pre- test and post-test). At the end of the treatment period, relevant post-tests are administered to the students of both the groups. The results of the study revealed that there were significant statistical differences at (a=0.09) among the grammar performance means of both groups at both levels due to inductive method.