Diversity-oriented synthesis of β-lactams was achieved via Ugi/Michael reaction cascades under mild conditions. The intramolecular hydrogen bonding between the heteroatom from an aldehyde component and the amide NH group controls the chemoselectivity of the Michael reaction versus the aza-Michael reaction. DFT calculation was performed to clarify the mechanism, chemo-selectivity and diastereoselectivity of this work. This one-pot protocol offers a straightforward method to build a diversified β-lactam library for drug discovery.