I went to the dermatology twice, told me theres nothing wrong about line in my fingernail. can you give me suggestion. ?

Second opinion: Get a second opinion from another dermatologist. If the second dermatologist says there is nothing wrong and you still worry about the line, i suggest seeing a psychiatrist.

Moniter closely: There are some lines (ie: melanonychia) which are normal and benign. Other lines are signs of melanoma or other disease. A dermatologist should be able to make the proper diagnosis. At the very least take a picture and moniter for change. Consider having a biopsy if the line changes or crosses the proximal nail fold (extends from the nail onto the skin).

Would need to see: what kind of line-is it pigmented? does it touch proximal cuticle. need more information, but most board certified dermatologist know when to biopsy