This study is intended to identify English pronunciation made by young learners. The objectives of this study are: (1) To Identify the pronunciation errors on vowels and diphthong made by the students of SEC student of SMA 1 KENDAL. (2) To find out the type of errors in pronouncing English vowels and Diphthong sounds experienced by the second-grade students of SEC student of SMA 1 KENDAL. The subject of this study is the member of SEC (Smanik English Club) academic year 2019/2020. The instrument used to collect the data is the pronunciation errors test consists of a word, phrase, and sentences taken from Mark Hancock, English Pronunciation in Use (self-study and Classroom use), Cambridge University Press. The test is intended to show the result of the students’ errors in producing vowels and diphthongs. In this study, the type of error type has been found, so for teachers it is very helpful so that it can be analyzed to solve pronunciation problems. The first type of errors is interlingual Error, Students pronouncing word as the student’s native language or first language. Then the second is intralingual errors, it means phenomenon in which language learner apply a rule or a pattern in a situation it does not apply in the target language. To complete the shortcomings of this study, further research is needed to find out the exact cause of students making errors in pronouncing vowel and diphthong.