From Longman Dictionary of Contemporary Englishnot that I know ofnot that I know ofused to say that you think the answer is ‘no’ but there may be facts that you do not know about ‘Did he call earlier?’ ‘Not that I know of.’ → know
Examples from the Corpus
not that I know of• "Did anyone call for me?" "Not that I know of."• Answer, not that I know of. 