For US:
Can I use my Nexus card instead of a passport?
Yes you can. Under the Western Hemisphere Travel Initiative (the States and Canada are members) the Nexus card along with the enhanced driver’s license, SENTRI pass, FAST card for truckers (road travel only) and US passport card are recognized as acceptable equivalent documents to a valid passport. There have been noted incidents where border guards have asked to see a passport as well as a NEXUS card, but under US law brought in on June 1, 2009 legally the above documents count for travel between Canada, Mexico, the Caribbean or Bermuda, by land or sea.

For Canada:
NEXUS cards are now accepted as proof of identification and to denote citizenship when entering Canada by land, air (when entering Canada from the U.S.), or marine ports of entry. This means that Canadian or U.S. citizens who are NEXUS members may use their membership card when entering Canada by land, air or boat when using non-NEXUS lanes. NEXUS members who are permanent residents of Canada or the U.S. are still required to travel with a passport and proof of permanent residence, and may be requested to present these documents to the officer upon arrival at the border.