Calculus in high schools is a discipline of advanced mathematics. Its intrinsic complexity is very high. Many researchers around the world have pointed out difficulties and obstacles that students have encountered when studying it. To understand a concept and a theorem of calculus is not easy; therefore, when solving calculus problems students cannot avoid errors. Find out the errors of students in mathematics learning have been a problem that several of mathematics educators around the world interested in. In this paper, we report the answers to the following two research questions: (1) In solving calculus problems, what errors have 12th grade - students committed? (2) What are the teachers' opinions about how often the errors of the students have occurred? The data collection was performed in high schools in Tan Chau town, An Giang province, Vietnam