An alignment method for assembling substrates without fiducial mark is provided and has steps of: pre-defining at least two partially standard character regions; capturing at least two partially actual images of a first substrate; comparing to obtain at least two partially actual character regions; building an actual coordinate system of the first substrate; comparing the actual coordinate system with a coordinate system of a second substrate to obtain three types of offset values; moving the first substrate to a correct waiting position based on the offset values; ensuring if the first substrate is disposed at the correct waiting position; and stacking the first substrate with the second substrate to finish the alignment and installation. Thus, the alignment method of the present invention can be applied to to-be-installed substrates without any fiducial mark for alignment.