Fractions remain one of the most difficult topics to convey to students. Thus, employing professional development programs known for improving the teaching and learning process, such as lesson study, is deemed necessary. This study aims to assess and analyze the strengths and weaknesses of the implemented research lesson using the Mathematical Quality of Instruction (MQI) rubric. This standardized observation tool evaluates the quality of a mathematical instruction. Results indicated that the strengths of the implemented research lesson were demonstrated in the fluent utilization of mathematical language under the domain Richness of Mathematics and the minimal instances of teacher's error and imprecision. On the other hand, weaknesses were observed in student participation under the domains of 'Common Core Aligned Student Practice' and 'Working with Students' in Mathematics. Implications for educators and the various educational processes include among others the importance of lesson preparation, teaching students to articulate ideas explicitly and the continuous use lesson study as a professional learning community.