The couple's union was rocked last year (09) when a stripper alleged she had sex with the Transformers star behind Fergie's back.
Duhamel denied the story and the couple went on to renew their wedding vows in January (10).
The actor admits their relationship has "changed so much" in the six years they have been together, but he's convinced their union is now completely solid.
He tells Britain's Company magazine, "(The love) ebbs and flows. Fergs and I have been together for six years and our relationship has changed so much. I mean, the first three months, well, there’s nothing like it! But, after six years, our marriage is stronger than ever, and it can get even better... I believe in love, for sure. I don’t think there’s any doubt that love is real, but there are different levels."