The　prospect　of　Motor　Imagery　(MI)　BCI　is　attracting　the　researchers　around　the　world.　For　MI　BCI,　training　a　user　is　a　difficult　and　time-consuming　task.　This　study　aims　at　finding　the　pattern　of　Event-related　Potentials,　by　which　we　can　improve　the　training　process,　during　training　MI　users.　We　designed　the　experiments,　acquired　the　EEG　signals　and　analyzed　them　during　the　periods　when　the　subjects　were　executing　the　training　trials　and　when　they　completed　the　training　trials.　The　results　show　that　the　obvious　potential　patterns　that　are　related　to　small　probability　events　exist　in　the　both　situations　and　the　elicited　potentials　on　frontal　lobe　are　commonly　stronger　than　ones　on　other　brain　areas.　We　speculate　that　Attention　Mechanism　is　deeply　involved　in　the　process　of　MI　training.　The　finding　would　underlie　our　future　work　intending　to　develop　the　new　MI　training　means　and　algorithm.　©　2017　IEEE.