The main objective of this study was to investigate the effect of explicit grammar instruction on the EFL students’ paragraph writing performance at Adet preparatory school. The writing aspects that make students effective in their writing performance were also investigated. The study was conducted on grade 11 EFL students. This study was conducted based on a quasi-experimental design: pretest-posttest design in which a method that used intact groups or the whole students in the class room rather than select participants randomly. In line with this, quantitative data about the students’ paragraph writing skills held with all participants. The participants were 98 students of two sections (M and N) selected from a population of 7 sections of 347 students. The two sections were selected using purposeful sampling technique and formed as control/ comparison and experimental groups by casting lottery. Since the collected data were quantitative in nature, the analysis method was quantitative technique as well. Therefore, Cronbach alpha was computed to measure the inter-rater reliability between the three raters. In the second procedure, independent and paired sample t-tests were also computed to determine the difference between the two groups and the students’ improvement in experimental group in their paragraph writing skills respectively. At the beginning of the study, the students in control and experimental groups were asked to write paragraphs without any training. Then, the students in experimental and controlled group were given training grammar explicitly and implicitly respectively for sixteen weeks and then they wrote paragraphs within the same time and on the same topic. The analysis of the students’ written texts (paragraphs) before and after training showed statistically significant mean score difference between the control and experimental group students in terms of aspects of writing (content, organization, cohesion, vocabulary, grammar, mechanics and spelling; among these cohesion (0.45)and content (0.22) showed the highest and the least improvements respectively). In sum, the results showed/ noticed that students in the experimental group outperformed the control group in their writing performance due to the explicit grammar teaching. Besides, the mean score of post-test writing is better than the pre-test writing of the experimental group. Generally, the findings indicated that teaching grammar explicitly could encourage the students to write effective paragraphs. Thus, the method could be used by English teachers in helping students to improve their paragraphs writing ability and skills.