The exfoliation of layered montmorillonite (MMT) into mono- or few-layer sheets is of significance for both fundamental studies and potential applications. In this report, exfoliated MMT nanosheets with different aspect ratios have been prepared via a new freezing/thawing-ultrasonic exfoliation method. Freezing/thawing processing can exfoliate MMT tactoids with low efficiency while virtually retaining the original lateral size. The ultrasonic method has better exfoliation efficiency but tends to damage the nanosheets. By combining them and reasonably controlling the cycle index of freezing/thawing and ultrasonic power, the MMT nanosheets with different aspect ratios have been prepared efficiently. Such a unique exfoliation method has broad applicability for layered materials to produce monolayer nanosheets on a large scale.