<Page> EXHIBIT 6 TRANSCANADA CORPORATION 450 - 1st Street S.W. Calgary, Alberta, Canada T2P 5H1 CERTIFICATION OF CHIEF EXECUTIVE OFFICER REGARDING PERIODIC REPORT CONTAINING FINANCIAL STATEMENTS I, Harold N. Kvisle, the Chief Executive Officer of TransCanada Corporation (the "Company"), in compliance with 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, hereby certify, in connection with the Company's Quarterly Report as filed on Form 6-K for the period ended December 31, 2003 with the Securities and Exchange Commission (the "Report"), that: 1. the Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and 2. the information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company. /s/ Harold N. Kvisle ----------------------- Harold N. Kvisle Chief Executive Officer January 26, 2004