Exhibit 5.1

                      [Letterhead of Davis Polk & Wardwell]

                                                                  March 24, 2004

Ultra Clean Holdings, Inc.
150 Independence Drive
Menlo Park, California 94025

Ladies and Gentlemen:

      Ultra Clean Holdings, Inc., a Delaware corporation (the "COMPANY"), is
filing with the Securities and Exchange Commission a Registration Statement on
Form S-1 (the "REGISTRATION STATEMENT") for the purpose of registering under the
Securities Act of 1933, as amended (the "SECURITIES ACT"), 6,000,000 shares of
common stock, par value $0.001 per share (the "FIRM SECURITIES"), as well as
900,000 shares of common stock, par value $0.001 per share, which may be sold by
the selling stockholder (the "SELLING STOCKHOLDER") subject to the
underwriters' over-allotment option (the "OPTION SECURITIES" and, together with
the Firm Securities, the "SECURITIES"), as described in the Registration
Statement.

      We have examined such documents and such matters of fact and law that we
have deemed necessary for the purpose of rendering the opinion expressed herein.
Based on the foregoing, we are of the opinion that:

          (i)  When the price at which the Securities to be sold by the Company
               has been approved by or on behalf of the Board of Directors of
               the Company and when the Securities to be sold by the Company
               have been duly issued in the manner described in the Registration
               Statement and delivered against payment therefor in accordance
               with the terms of the Underwriting Agreement referred to in the
               Registration Statement, the Securities to be sold by the Company
               will be duly authorized, validly issued, fully paid and
               non-assessable.

          (ii) The Securities to be sold by the Selling Stockholder have been
               duly authorized and validly issued and are fully paid and
               non-assessable.

      We hereby consent to the filing of this opinion as an Exhibit to the
Registration Statement referred to above, and further consent to the reference
to our name under the caption "Legal Matters" in the Prospectus which is a part
of the Registration Statement, without admitting that we are experts within the
meaning of the Securities Act.

                                        Very truly yours,

                                        /s/ Davis Polk & Wardwell