Improvement of a drug's binding activity using the conformational restriction approach with sp³ hybridized carbon is becoming a key strategy in drug discovery. We applied this approach to BACE1 inhibitors and designed four stereoisomeric cyclopropane compounds in which the ethylene linker of a known amidine-type inhibitor 2 was replaced with chiral cyclopropane rings. The synthesis and biologic evaluation of these compounds revealed that the cis-(1S,2R) isomer 6 exhibited the most potent BACE1 inhibitory activity among them. X-ray structure analysis of the complex of 6 and BACE1 revealed that its unique binding mode is due to the apparent CH-π interaction between the rigid cyclopropane ring and the Tyr71 side chain. A derivatization study using 6 as a lead molecule led to the development of highly potent inhibitors in which the structure-activity relationship as well as the binding mode of the compounds clearly differ from those of known amidine-type inhibitors.