>Yes, since I wnat to store 13 digit number which can be use by numeric 13 or double. But double stores in 8 bytes, while Number will take 13 bytes. Is it overkill? Is it link with corruption of index?

VFP attempts to do index key compression. Storing keys as doubles MAY not work as well as character or integer if VFP fails to find a pattern in the binaries. You may have a smaller DBF, but may wind up with a bigger CDX. A few tests might be in order.