Yes, you can join B.Com after completing a diploma, but it depends on a few important factors:

Diploma After 12th (10+2):

If your diploma was done after completing 12th grade (HSC), then you are eligible for B.Com in most universities.

Diploma Equivalent to 12th (10+3):

Some diplomas (like Polytechnic diplomas) done after 10th are considered equivalent to 12th by some universities.

IT IS NOT POSSIBLE IF:

If your diploma is not equivalent to 12th, or the university does not recognize it for undergraduate admission, you may be denied entry to B.Com.