This study is focused on the effectiveness of the use of Mobile Learning Device (MLD) (by using the “String” application) to the learning outcome among the Junior Students in De La Salle Santiago Zobel School in wave concepts. This study is only limited to selected students in the Third Year Level Physics classes which is composed of one (1) section only namely: Junior I. The study will involve a total of thirty nine (39) students from Third Year Level section Junior I. The group consists of twenty three (23) male and sixteen (16) female students. The selected students are among the pioneers of the PEARL program. The program aims to directly respond to the skills development of the 21st Century Learner and is inspired by the innovative attitude of the Founder of the De La Salle school system, St John Baptist de la Salle (France, 1719). Students enrolled are going to utilize Mobile Learning Device (MLD) as a tool in the teaching and learning process. In line with the findings of the study, the researchers arrived at the following conclusions: There was a significant difference between the pre-test and post-test results using the mean scores of the students in Junior I which are 4.33 and 10.13 respectively. The data revealed in the test of the level of significance that the obtained t-value 4.276 was higher than the critical t-value 2.145 with significance level at 0.05; thus, the null hypothesis was rejected and interpreted as significant. Since the mean scores of the students of Junior I were higher in the post-test than in the pre-test, it was evidently seen that there was an improvement in the performance in concepts of waves using Mobile Learning Device application String.