Exhibit 5.1

April 5, 2001

Avant! Corporation
46871 Bayside Parkway
Fremont, CA 94538

Registration on Form S-3

Dear Ladies and Gentlemen:

    We have acted as counsel to Avant! Corporation, a Delaware corporation (the "Company"), in connection with its registration statement on SEC Form S-3 of the public offering of up to 2,000,000 shares of common stock, par value $0.0001 per share (the "Common Stock"), to be offered for resale by the selling stockholder identified therein.

    In this regard, we have examined the Company's Restated Certificate of Incorporation and Bylaws, each as amended to date, and records of meetings of and written consents by the stockholders and the directors of the Company. In addition, we have made such examinations of matters of law as we deemed appropriate for purposes of this opinion. As to certain factual matters we deem relevant to this opinion, we have relied upon a certificate of officers of the Company and have not sought to independently verify the matters stated therein.

    Based upon the foregoing, it is our opinion that the 2,000,000 shares of Common Stock have been validly issued, are fully paid and non-assessable, and no personal liability will attach to the holders of such shares by reason of the ownership thereof.

    This opinion is rendered solely to you in connection with the registration of the shares of Common Stock under the Registration Statement.

    We consent to being named as counsel to the Company in the registration statement and to the inclusion of a copy of this opinion letter as an exhibit to the registration statement. In giving this consent, however, we do not thereby admit that we are an "expert" within the meaning of the Securities Act of 1933, as amended.

Very truly yours,
MCCUTCHEN, DOYLE, BROWN & ENERSEN, LLP
By:	/s/ ALAN B. KALIN Name: Alan B. Kalin Title: A Member of the Firm