Can you patent a business model?

Like an assembly line or something like that?

No I don’t think so.

more like for an online internet business model (or process)

Thats an interesting question, especially since this has recently been asked to the Supreme Court in In re Bilski. I’m uncertain if they have accepted the case yet, but they likely will. Essentially, a business method may be patented, however it must be (1) tied to a particular machine or (2) that transform an “article” into a different state or thing. I suggest speaking with a patent attorney for applying these concepts to your idea to determine patentability.