The 12 MeV electron beam reference depth zref at three field size FS 10 × 10 cm2, 15 × 15 cm2, 20 × 20 cm2 were determined for TRS277 and TRS398 IAEA protocols. At a fixed SSD = 100 cm, results showed that all zref TRS398 are higher than zref TRS277, in the range of 0.01 to 0.10 cm. Reasons for this is explained. For a particular zref of TRS398 at FS 10 × 10 cm2, comparison was made with earlier results obtained by other worker that used similar experimental set-up except a different linear accelerator. The present result is comparable since it is higher only by 0.11cm.