The aim of this study is to examine if using WhatsApp actively in English outside the school has any effect on the students' attitudes and self-efficacy belief on English course. In this study, a sequential explanatory pattern was used. In order to obtain quantitative data, one group pretest-posttest design was used. The participants consist of 11th grade students studying in an Anatolian High School in the center of Sivas. The implementation of the study lasted eight weeks. In terms of the experiment, WhatsApp meetings were held twice a week for evaluating the achievement of 11th grade English course. "English Self-Efficacy Scale" and "Attitude toward English Scale" were applied before and after the experiment. In addition, semi-structured questionnaire was used in order to get the opinions of the students about the implementation at the end of the experiment. The results of the study revealed that using WhatsApp outside the course leads to a significant differentiation in the self-efficacy beliefs of the students for both reading and listening. The data obtained from qualitative processes also confirm this. In addition, it shows that the practice has a statistically meaningful effect on students' attitudes toward the course. The data obtained from the qualitative processes have proved that the implementation is influential in the emotions such as happiness, joy, excitement, pride and that the students considerably support the use of this implementation.