I think odt files are ziped files.
Why odt file can not be opened by zip but can be opened by open office?
And a zip file not complete can also be unziped??
Who is familiar with zip format?
ODT files may or may not be zipped. It could also be a plain basic XML file (see http://en.wikipedia.org/wiki/OpenDocument#Specifications).
Also, there are several versions of the ZIP file format that may or may not inter-operate well, so maybe it's just your ZIP utility not being able to read the particular format your ODT file uses.
Rename the file's extension to zip. In that case, it should open with the zip unarchiver (tested with 7-Zip, PowerArchiver, Total Commander).
The problem seems to lie with the archivers' extension sniffing: PowerArchiver (albeit an old, old version) had problems opening the file when the extension was odt, but worked fine with zip.
Of course, the file extension odt will probably be associated with OpenOffice, so double-clicking the file will start OpenOffice.
Finally, i found the file is doc format, but saved as odt. This file is searched by google.
odt files are open office document text files.