The present disclosure provides a method includes forming a multi-fin device. The method includes forming a patterned mask layer on a semiconductor substrate. The patterned mask layer includes a first opening having a first width W1 and a second opening having a second width W2 less than the first width. The patterned mask layer defines a multi-fin device region and an inter-device region, wherein the inter-device region is aligned with the first opening; and the multi-fin device region includes at least one intra-device region being aligned with the second opening. The method further includes forming a material layer on the semiconductor substrate and the patterned mask layer, wherein the material layer substantially fills in the second opening; performing a first etching process self-aligned to remove the material layer within the first opening such that the semiconductor substrate within the first opening is exposed; performing a second etching process to etch the semiconductor substrate within the first opening, forming a first trench in the inter-device region; and thereafter performing a third etching process to remove the material layer in the second opening.