The study concerned with discussing about slang words used in Bruno Mars song lyrics. The research design of this study is descriptive qualitative method. In this study, the theory of word-formation process by George Yule process (1986) in his book “the study of language” is used to analyze the formation process of each slang word. The researcher only analyzes five in Bruno Mars song lyrics. They are count on me, show me, the lazy song, liquor store blues and just the way you are. There are consists of two research problem : (1) what is the type of slang word used by Bruno Mars in lyrics, (2) what is the dominant slang word found in Bruno Mars song lyrics. The data were document analysis, the research tried to find and identity the slang word in lyric by marking, finding and analyzing the slang word. The result shows that from 58 data taken, not all process of slang-word formation appear in the lyrics. Five process of slang-word formation found in the lyrics. The process of slang word formation that are used in the lyrics are clipping(160%), blending(120%), coinage(80%), borrowing(0%), and acronym(60%).