The purpose of this study is to determine the effects of the multimedia annotations which were sent through instant messaging application WhatsApp on students' success in English vocabulary acquisition together with their opinions on the educational use of WhatsApp and the multimedia annotations. In the study, mixed method model, which combines both quantitative and qualitative data, was employed. Results of the variance analysis of the quatitative data indicated that the multimedia annotations sent through WhatsApp yielded a meaningful increase in students' vocabulary acquisition. Multiple comparison test to identify the reason of this increase revealed that Text+Picture+Audio' and Text+Picture' multimedia annotations increased students' vocabulary acquisition meaningfully when compared to other learning multimedia annotations. Although Text+Audio' multimedia annotation contributed to the success of students more than the students who received only 'texts', it wasn't found meaningful. In the qualitative aspect of the study, students expressed their willingness to go on the treatment. They stated that similar researches in their other courses could be useful. They also added that they could learn unconsciously and expressed positive opinions towards the use of the pictures and audio annotations in particular. However, a few students reacted negatively againts the irrelevant posts and stated that the posts occupied a lot of space on their phones.