Despite the promising functions of a multi-function hand, it is challenging to learn to use a hand that has up to 36grip patterns. If it requires too much cognitive load to learn to operate a prosthetic hand, the user may eventually stopusing it. Measurement of cognitive load while learning to use a bionic hand will help the therapist to adjust the trainingpace and help the user to achieve success.An innovative, non-obtrusive method for measuring cognitive load is by tracking eye gaze. Gaze measuresprovide pupil diameters that indicate subjective task difficulty and mental effort. Three subjects wore a pair of Tobiieye-tracking glasses during control training and performed eight activities. Eye-tracking data were imported in TobiiPro Lab software for extracting pupil diameter during the activities. Pupil diameter (normal range: 2-4mm duringnormal light) was used to indicate the amount of cognitive load.Pupil diameters were below 4mm in 9 out of 23 training activities. Pupil diameters were above 4mm in all threesubjects when they used precision pinch to perform the activities “stack 4 1-inch wooden blocks” and “pick up smallobjects”. Subject 3 had pupil diameters over 4mm in all training activities. Pupil diameters were largest when thesubjects were adjusting the grip and when they had difficulties in initiating the grip.It seems appropriate to introduce no more than four grips during the first control training session. Further studyis required to determine if pupil diameters will decrease over time when adequate prosthetic training is given.