I am btech mechanical engineer having work experience in india....I have my relatives in calgary,who owns an auto service centre.Could they offer me a job letter and would that be accepted under LMIA?
Hard to say. They will have to prove they were unable to find a Canadian with the right skills for the role (which may be challenging to do given Calgary's current unemployment numbers).
A job offer letter isn't enough. They will have to apply for and obtain an approved LMIA before you can get 600 points under Express Entry (the job will also have to be NOC A, B or 0).