The purpose of this study was to explore the problem-solving performance of sixth-grade elementary school students in the unit of ratio and proportion, explored the error patterns that students often made in this unit. This study adopted paper-and-pencil tests to collect quantitative data. The sample consisted of 237 students from thirteen classes of eight elementary schools in Tainan City. The students were tested by the researcher’s self-edited "Ratio and Proportion Achievement Test."Subsequently, organized the error patterns made by students in the ratio and proportion unit, and found out the possible reasons of the error patterns. The research results are as follows: 1.The performance of sixth-grade elementary school students in the ratio and proportion unit from the entirety and different dimensions was different. All of the students had the higest grade in the equivalent ratio dimension, and had the lowest grade in the application of the ratio dimension. 2..There were seventeen error patterns that commonly made by sixth-grade elementary school students in the unit of ratio and proportion: Misunderstanding the question, misplacing antecedent or consequent of the ratio, unclear concept of antecedent and consequent of the ratio, unclear concept of proportion, confusing concept of ratio with concept of proportion, unclear concept of equivalent ratio, unclear concept of irreducible ratio, not converting ratios into the lowest term by insufficient knowledge of irreducible ratio, solving the problem by using one or multiple steps, ignoring the meaning of the question, the wrong or incomplete of the equation, the incorrect formula of calculation, transferring the unit incorrectly, the incorrect unit, not calculating process, not writting the answer of the application questions, and the incorrect calculation. 6.There were eight possible reasons for the error patterns made by sixth-grade elementary school students in the unit of ratio and ratio: Unclear concept of the whole quantity or part quantity, confusing concept of ratio with concept of proportion, misunderstanding the question, unclear the calculation rules of the ratio, unclear concept of ratio, insufficient knowledge of reading comprehension, insufficient of basic arithmetic ability, and writing carelessly.