Thanks Jbswain for your reply.
I am afraid whether I could understand your advice fully, but I am trying to run Watershed tool of Hydrology tools. The raster "flowdir01" is an input raster of Watershed tool. When I run Watershed, I have received the following error message:
Messages
Executin: Watershed flodir01 pourpoint01 C:\GISdata\Watershed01 Id
Start time: Thu Oct 03 09:02:03 2013
ERROR 999999: Error executing function.
The name of the Field is invalid: valid names may contain letter, numbers or underscores.
ERROR 010419: Unable to build the attribute table of C:\GISdata\Flowdir01 as required by the operation.
Failed to execute (Watershed).
Failed at Thu Oct 03 09:02:13 2013 (Elasped Time: 10.00 seconds)
Before that, in the properties of the raster layer, flowdir01, when I selected "Show: Unique values" at Symbology tab, "Build Raster Attrribute Table" windows appears saying that "Raster attribute table doesn't exist. Do you want to build attribute table?" I have replied "Yes" to this message. After a few seconds, the cursor returned to "Show: Classified". After that, when I "Show: Unique values", the above message always seem to appear, and I am a little confused whether this message really meaning what it seems to say, or raster attribute table is not properly build.
From the error message of Watershed above, the raster attribute table seems to be build after the process above, but may be not appropriately build as described in ERROR 999999, the field name is invalid. If this is correct, please advice me the methods to check this error and to repair this error. Thanks for your replies in advance.