The ratio is a mathematical concept that is useful in real life. In Vietnam, at an elementary level, students are acquainted with ratios in grade 4 through word problems and arithmetic operations. In grade 5, students continue to be familiar with proportional quantities and percentages which are related to ratios. In secondary schools, students will continue to study the slope of a straight line and trigonometric concepts which are essential applications of the ratio. Therefore, it is essential to help Vietnamese students understand the concept of a ratio and successfully solve word problems involving ratios. It should be an indispensable objective in teaching mathematics at primary levels in general and in grades 4-5 in particular. The paper presents the results of the analysis of mathematical content related to the ratio in primary mathematics textbooks of Vietnam and an experiment on solving word problems related to ratios of the fifth-grade students. It was found out that the misconceptions of ratio and lack of solving strategies cause primary school students' difficulties in solving word problems involving ratio. From that, the authors propose some solutions to improve students' abilities in solving these problems.