This research is to study how the two mathematics problem-solving strategies, length and proportional line-diagram representations, affect sixth grade students’ cognitive load and learning performance during they study the unit of “baseline and comparison”. The researchers adopted Schnotz’s integrated model of text and picture comprehension to analyze the cognitive process of studying worked examples with line diagram, and compare the differences in term of cognitive load between the two methods based on cognitive load theory. Then, the researcher conducted a three phases of true-experimental design to show the effectiveness of our analysis. The experimental results show that the proportional line-diagram condition performed significantly better than the length line-diagram condition on post-tests, and after the students had learned the unit of “ratio”, the difference between the length line-diagram condition and the proportional line-diagram condition on delay-test are not significant. Finally, the researcher will provide some implications for the educational authorities, teachers, and future researches.