In many type Ⅱ solar radio bursts, the fundamental and/or the harmonic branches of the bursts can split into two almost-parallel bands with similar spectral shape and frequency drift. However, the mechanisms accounting for this intriguing phenomenon remain elusive. In this presentation, we report our latest progresses on both case and statistical observational studies of band-splitting type IIs. We will provide evidences which seem to be inconsistent with the popular upstream and downstream scenario.