A new series of [1,2,4]triazolo[1,5-a]pyrimidine-based LSD1 inhibitors were designed, synthesized, and further evaluated for their cytotoxicity against MGC-803, EC109, A549 and PC-9 cells as well as the ability of inhibiting LSD1. Some of these compounds showed potent inhibition toward LSD1 and selectively inhibited growth of A549 and PC-9 cells. Compound 6l potently inhibited growth of PC-9 cells (IC50 = 0.59 μM), about 4-fold more potent than 5-FU. Further SARs studies led to the identification of compounds 6l-m, which had good growth inhibition against all the tested cancer cell lines and were much more potent than 5-FU and GSK2879552. Besides, compounds 5p, 5q and 6i inhibited LSD1 potently (IC50 = 0.154, 1.19 and 0.557 μM, respectively). Docking studies revealed that compound 5p formed arene-H interactions with Val333 and hydrogen bonds with surrounding Ala331, Met332, and Ala539 residues. Compound 5p significantly inhibited migration of A549 and PC-9 cells in a concentration-dependent manner, but had different effect on the expression of E-cadherin and N-cadherin. The [1,2,4]triazolo[1,5-a]pyrimidine scaffold may serve as a starting point for developing potent LSD1 inhibitors for cancer therapy.