Did you use ENVI's QuickBird radiance tool to convert the DN to radiance?  Did you leave the default parameter 'Scale Output to Integers' to 'Yes' so that the output radiance values are scaled by 1000?  If so, did you enter 1000 as the scale factor in FLAASH?  An incorrect scale factor is often the issue when you get unexpected reflectance results.  However, it is not uncommon to get some pixels that are negative reflectance.  If you have a very low radiance value (for example, a pixel in shade), then FLAASH may not be able to model the reflectance accurately and you can get some negative reflectance values.  It just means for those pixels, the results are unreliable and therefore should be excluded.

Thanks a lot mminari!. Its work. I still got negative values but it is now acceptable. Just a small question, you mentioned that the scale factor for FLAASH in your reply is 1000. However, the default value is 10,000. Which one is right? Anyway, read somewhere in this forum that, if I used 10,000 as FLAASH scale factor, i need to divide my reflectance with 10,000.00 in order to get actual reflectance. So if I used 1,000, then I need to divide my reflectance to 1,000. Right?  Thanks for your prompt reply. Really appreciate it.

Yes, you are correct.  The output reflectance is scaled by 10000.  I left off a 0! :)