This paper is analyzed the differences between North and South Korean vocabulary and it’s taken aim at the causes of those differences, through comparisons and test surveys between North and South vocabulary. First of all, to research from the vocabulary derivation, it is found that the native tongue is occupied the highest percentage in both North and South, and there are more sino-Korean words in North than that of South. And as expected, there are more foreign words and Hybrid words in South than that of North. Second, through the vocabulary test, it is found that there are some differences among the basic vocabulary and it accounts for 20~21 percent of the whole basic vocabulary. The younger generation in South knows less vocabulary of North, while the younger generation in North knows more vocabulary of South. Finally, it is divided into three factors which causes these differences. The difference caused by new appeared vocabulary is the largest in differences between south and north vocabulary, especially, the difference caused by foreign words are the strongest. There also exists the difference of spelling, but it doesn’t influence on understanding of the vocabulary.