Provided are apparatuses and methods for diagnosing failure of a current sensor of a battery system for an eco-friendly vehicle. One apparatus for diagnosing failure of a current sensor of a battery system for an eco-friendly vehicle includes a high voltage battery, a current sensor configured to measure a current output from the high voltage battery, and a controller configured to measure an offset current through the current sensor when an ignition key is turned off or turned on, and diagnose failure of the current sensor on the basis of the measured offset current.