This study presents the improvement produced by the application of metacognitive instruction in simultaneous processing in grade 3 students of primary education. The sample consisted of 30 students from a classroom in a primary school. Two groups were formed, the intervention group was formed with the students of the classroom who presented difficulties in the simultaneous processing, showing a cognitive weakness that compromises the reading comprehension. The second group consisted of the rest of the students in the group, constituting the comparison group. A pre-post-follow-up test design is applied. The groups are matched in socio-economic and cultural status and in the successive processing. They differed in the simultaneous processing. The intervention group was applied the operative metacognitive instruction program through the planning facilitation strategy, which allows the students to answer the comprehension texts and also verbalize the reflex to compare and contrast their answers. The comparison group followed the curricular activities according to the traditional teaching model. The program is written in 24 sessions for seven weeks. After the intervention, the group improved significantly in the simultaneous processing. These improvements are interpreted in the sense that the metacognitive instruction that facilitates the discussion between the students and the reflexive verbalization to compare and contrast answers, is responsible for the improvement of the simultaneous processing function for reading comprehension.