Nowadays Augmented Reality (AR) is very popular in teaching and learning, especially in leading student to observe learning material targets which supported by online learning resources using mobile device and wireless technology. Because English is a foreign language, most students have problems in learning English. Generally, their problems were related to low motivation, bad study habit and attitude toward learning. Significantly, formative assessment is important in improving student learning achievement. furthermore, a formative assessment can help teachers to identified students' progress, modify instruction, provide immediately feedback and achieve learning goals. The aim of this study was to build Mobile Based Assessment with Augmented Reality based on formative assessment mechanism, examine student's learning achievement, student's acceptance and student's motivation in Basic English Learning. A total of 34 elementary students of Grade 4 were selected to participated in this study. The descriptive analysis was used to analyze and described the data obtained. The result indicated that using Mobile Based Assessment with Augmented Reality based on formative assessment mechanism improved not only student's learning achievement but also student's motivation in Basic English Learning.