Try this

In a Layer 2, I added a Layer Mask , and I used the gradient tool (black & White ) to fill it . I think something in Layer 1 will appear ( the area filled with Black ) . But it likes nothing happen I can't even use the Gradient tool in Layer Mask . How do I do to be able to use the gradient tool in a Layer Mask ???
Any reply would be appreciated

Your question was asked very oddly, but I will try to answer it as well as I can.

Using a layer mask on an empty layer 2 will result in nothing showing up or disappearing. You have to actually have something on the layer in order to mask it out.

If you are trying to mask something on layer 1, you add the mask to layer 1, not a layer above it.

Once you add the mask to a layer, photoshop automatically switches to mask editing. Turning on the gradient fill tool and using black and white will result in a smooth transition from opaque to transparent pixels. You can switch from editing the mask to editing the graphic on the layer by using your mouse and clicking on the mask icon that was added to your layer on the layers palette.

BTW, you cannot add a layer mask to the lowest/background layer. You *can* add a mask to it if you change the background layer into a standard layer by double clicking it on the layers palette.

Thanks for your reply . Finally , I got the answer . The Blend Mode in default is Screen , but we MUST change it to Normal ( am I silly ? ) . That is .
Anyway , thanks