I met a problem: when I use the multi-material in my model--two fluids divided by a baffle, the geometry writing will give me an error: "adjacent cells are in different fluid streams and not separated by a wall or baffle."

You know, I used the baffle, that is why I was pussled. I even used wall to separate the two type of materials. No use. WHo can give me an advice?

It means a baffle cell, not a baffle boundary condition.

If you use separate fluid streams (ie different materials) then they cannot touch each other anywhere. There MUST be a baffle cell separating every location where the 2 streams would otherwise touch. If you are getting the error message, it means that somewhere your 2 fluid streams are directly connected.