A function diagnostic system for a filter of an internal combustion engine may be provided. The function diagnostic system may include a PM sensor and an electronic control unit. The electronic control unit may be configured to: calculate a NOx conversion rate in the filter; determine whether a NOx converting function of the filter is normal; execute a sensor regeneration process for removing the PM deposited between an electrodes of the PM sensor; continuously monitor an output value of the PM sensor after a predetermined PM deposition restart time, after execution of the sensor regeneration process is finished; and determine that there is an abnormality in a PM trapping function of the filter, when the output vale of the PM sensor, which has been continuously monitored, is reduced when the electronic control unit determines that the NOx converting function of the filter is in a normal status.