Three symmetric linear pentablock terpolymers, poly(tert-butyl acrylate-b-styrene-b-ethylene oxide-b-styrene-b-tert-butyl acrylate) (TzSyOxSyTz)) with similar lengths of PS (S) and PtBA (T) blocks on the two sides but various lengths of the PEO (O) block in the middle, namely, T95S62O46S62T95, T105S60O90S60T105, and T107S65O136S65T107, were synthesized from their corresponding parent triblock copolymers poly(styrene-b-ethylene oxide-b-styrene) (SyOxSy), that is, S62O46S62, S60O90S60, and S65O136S65. The phase behaviors of TzSyOxSyTz pentablock terpolymers were studied using synchrotron small-angle X-ray scattering and transmission electron microscopy. The PEO block length was found to significantly affect the phase structures of the obtained TzSyOxSyTz pentablock terpolymers. Although all three SyOxSy parent triblock copolymers were in disordered states, very interestingly, ordered structures were formed in all three TzSyOxSyTz pentablock terpolymers. Particularly, the double gyroid (Q(230)) network structure was experimentally observed for the first time in a symmetric linear ABCBA-type pentablock terpolymer, that is, T105S60O90S60T105. Moreover, the hexagonal perforated lamellae (HPL) structure was also observed in unsheared T107S65O136S65T107. Temperature-variable SAXS showed that T95S62O46S62T95 maintained an ordered lamellar phase structure in the temperature range of 25-210 degrees C, and an order-to-disorder transition occurred for T95S62O46S62T95 at around 170 degrees C. The HPL phase of T107S65O136S65T107 still partially remained with decreased ordering at a temperature of up to 210 degrees C.