The main aim of this research is to examine the effect of an instructional design based on the Algo-Heuristic Theory of Instruction (AHTI) on students' mathematics performance and logical thinking skills in the subject of 7th grade mathematics. The research is grounded in a mixed method. In the quantitative part of the research, a quasi-experimental design including pretest, posttest, retention test and retention monitoring test with experimental, placebo and control groups is used, while the qualitative section is based on a case study. The study group of the research consisted of three classes at a secondary school having a "medium" level of achievement. The quantitative and qualitative data of the study revealed that with the effect of the instructional design based on the AHTI, the mathematics performance of students in the experimental group increased, and that their logical thinking skills were positively impacted. Furthermore, it was revealed that students in the experimental group were able to transfer the stages of the Landamatics algorithm that they learned in the unit named "Ratios and Proportions" to their performance in science.