Is "avoiding eye contact" a sure sign of autism? or could it be a normal thing for 1.5 years child?

Could be normal: As they say, there are no "pathognomic signs" -- nothing which definitively establishes that a person has this or that disorder. So it could be normal; it could be not normal but not autism; or it could conceivably be autism. However, it would be unwise to be concerned that avoiding eye contact alone, as an isolated feature, points to autism, when it may be, as you suggest, nothing.

Possible: avoiding eye contact is a feature of early autism, but more data is necessary before making that diagnosis. There are shy children who do not have autism who also avoid direct eye contact. Address your concern with your pediatrician who may use an autism scale or refer you to a psychologist who deals with autistic children who can evaluate your child.