In eukaryotic cells, genomic DNA is hierarchically packed into chromatin by histones in the nucleus. Despite of over 30 years' study, the fundamental structure of 30 nm chromatin fiber remains controversial. In this review, we focus on recent progress in revealing the structure of 30 nm chromatin fiber, and emphasize on the most recent left-handed double helix structure of 30 nm chromatin fiber. In addition, we discuss the epigenetic regulation of the 30 nm chromatin structure. Finally, we discuss the possible challenge in elucidating the structure of 30 nm chromatin fiber and its regulation.