Alpha problem

Hi.
I am rendering a scene into an RGBA image;
however, I find the alpha value of the pixel with no object overlapped is 128, not tansparnt 0. may i know why? and how can I set it to 0.

Thanks in advance

At a guess:

glClearColor(0.0, 0.0, 0.0, 0.0);

Yes, it works
Thanks