This study identified the strengths and weaknesses of Korean high and low performers in science, comparing their performance with Canadian and Taiwanese students. Using raw data from the 2018 Programme for International Student Assessment (PISA), correct answer rates were analyzed for the Competency and Knowledge dimensions within the PISA framework. Additionally, correct answer rates for each item were examined across high and low performers in the three countries. The findings are as follows. In Competence dimension, both Korean high and low performers showed a weaker evaluation and design of scientific inquiry and a stronger scientific interpretation of data and evidence compared to the analyzed countries. In Knowledge dimension, Korean high performers and students as a whole showed stronger procedural knowledge, biological system, and Earth and space system content knowledge, and epistemic knowledge. Item characteristics with high performers’ lowest correct answer rate and low performers’ highest correct answer rate by analyzed country were as follows. Korean high performers showed difficulty in item types with more than one correct answer and items that required epistemic knowledge to answer the question. low performers showed difficulty solving familiar items with frequently encountered problem situations and had difficulty with items related to the ecosystem.