A latch-up prevention structure and method for ultra-small high voltage tolerant cell is provided. In one embodiment, the integrated circuit includes an input and/or output pad, a floating high-voltage n-well (HVNW) connected to the input and/or output pad through a P+ in the floating HVNW and also connected to a first voltage supply, a low-voltage n-well (LVNW) connected to a second voltage supply through a N+ in the LVNW, a HVNW control circuit, and a guard-ring HVNW, where the first voltage supply has higher voltage level than the second voltage supply, guard-ring HVNW is inserted in between the floating HVNW and LVNW to prevent a latch-up path between a P+ in HVNW and N+ in LVNW by using the HVNW control circuit that controls the guard-ring HVNW's voltage level. The guard-ring HVNW's voltage level is matched by the floating HVNW's voltage level.