The regioselective unilateral additions of Grignard reagents to acyclic or cyclic β-diketones were effectively promoted by sub-stoichiometric amounts of i-PrMgCl to afford β-tertiary hydroxyl ketones or 3-substituted cyclic-2-enones, respectively. Also, the addition of Grignard reagents to acyclic β-diketones followed by a reaction with cyclic β-diketones in a one-pot process was put forward. The reaction mechanism was discussed in detail to explain the high regioselectivity via chemical experiments, hydrogen-deuterium exchange and mass spectrometry.