B-Raf kinase plays a critical role in the Raf-MEK-ERK signaling pathway and inhibitors of B- Raf could be used in the treatment of melanomas, colorectal cancer, and other Ras related human cancers. We have identified novel small molecule pyrazolo [1, 5-a] pyrimidine derivatives as B-Raf kinase inhibitors. Structure–activity relationship was generated for various regions of the scaffold to improve the biochemical profile.