This study focuses on comparing two languages which are Japanese slang and Indonesian slang in morphology. The method used in this study is descriptive with qualitative approach and data analysis techniques is contrastive analysis. Contrastive analysis aims to identify differences and similarities between the two languages. Through contrastive analysis, the characteristic of each language will be obtained and it will reveal cultural differences between the first language and the second language. The data in this study were taken from one of popular social media, Instagram. The data were the captions and comments written by Instagram users in the range of age 15-30 years old. There are two objectives of this study. The first is to find out the meaning of slang in Japanese and Indonesian. The second is to find out the differences and similarities between Japanese and Indonesian slang especially from the word formation process. The results of this study found that there are many types of slang and the formation process. In Japanese slang, there are 43 types of slang and six types of formation processes. The formation process in Japanese slang is affixation, conversion, reduplication, blending, clipping, and initials. Meanwhile, in Indonesian slang, there are 51 types of slang and eight types of formation processes. The process of forming Indonesian slang is affixation, reduplication, composition, conversion, clipping, blending, acronym and initials and transposition. There are several differences in the process of forming Japanese and Indonesian slang. For instance, in Japanese slang, clipping occurs in the last two words and then are combined, while in Indonesian slang clipping occurs randomly in syllable leaching. Composition, transposition, and infix only occur in Indonesian slang, while the equation in the process of formation occurs in the affixation, conversion, reduplication, blending, clipping, acronym and initials process.