Evidence is a complicated subject, and it depends on what context you mean -- are you intending to tape someone in deposition, tape someone in a public place, tape someone at a place where they have an expectation of privacy, or something else?
if you mean taping someone while they're in a deposition, California allows taping of depositions as long as prior notice is given of the taping, and that tape can be used to impeach that witness at trial. If you want to use the tape at trial in lieu of live testimony, then more complex rules apply.
Assuming the tape is material, its admissablilty can depend on whether its properly authenticated by the author of the tape and the conditions under which it was made.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.