Hi,  I wanted to ask this question for long time.  So which likely to lose signal  1st?

If you are using 2.4ghz rx and 5.8ghz vtx, most likely vtx will lose signal first.

It depends on the power, and frequency. Given the same power, higher frequency usually gives up first.

ok thanks all.. that will clear it

This is a good question and an important consideration when planning a build and what components you are going to use, as you definitely want to lose video before receiver. It's bad enough losing your beloved quad in a flyaway, but it's worse having a first-hand view of it as it tries to make it's way back to China.