I'm very new supervisor, and need help about employee self-evaluation. Can you give some guidance about this?

Well I've never known an employee to make themselves look bad, so of course they are going to rate themselves higher on average. If the employee is always late, however, but rates himself "high" in the punctuality area, then I would definitely discuss it with him/her and discuss ways to improve. If he/she is substantially low in performance, you need to take another look at how the lower evaluation score was obtained. Is the score based on definite criteria, or is it subjective? Are you being fair in your evaluation? Are there any outside sources that are influencing the outcome of the evaluation vs. work performance alone? The purpose of evaluations is to find ways to help your employees succeed and to be productive members of the collective team - not to put them down and lower the morale any further. You want your employees to WANT to work for you.

I would have a 1 on 1 Interview with your employee and discuss some of his answers and why he feels his providing your company higher performance. Test his honesty. In today's world the most important thing in our job is people. If you trust them and they trust you. You have a company.

An evaluation is how you see an employee, not how he/she sees him/herself. Rate them how you see them, but have explanation of hows and whys ready in case HR gets involve.