1. I don't approve of this behavior.
2. I don't approve this behavior.

Which one is correct?

Question

1. I don't approve of this behavior.
2. I don't approve this behavior.

Which one is correct?

Answer

I agree, the first one is correct. However I wanted to let you know that APPROVE can be confusing.   Examples:

My professor approved of my proposal.   =  My professor thinks my proposal is good.
My professor approved my proposal.   =   My professor, who has the power to decide whether or not I do my proposal, thinks it is good and is allowing me to proceed.

Answer

First one is correct, Daniel.