The isothermal oxidation experiments were carried out on several new γ/γ′-strengthened cobalt-base alloys Co–Al–W–4Cr–0.02X (X = La, Ce, Dy, Y) at 900 and 800 °C. Due to an appropriate content of additional elements, the change in the morphology occurred and it significantly improved the oxidation resistance compared with those without Cr, among which the one with La elements shows the best oxidation resistance. Multiple oxide layers are also formed during the oxidation process, with CoWO4 and CoAl2O4 phases in the outer layer, and Cr, Al, W, and Co (e.g., Cr2O3) in the middle layer. The inner layer consists of some Al2O3 oxides, while more protective Al2O3 oxide was formed, esp. at the temperature of 800 °C. Both Cr2O3 and Al2O3 oxides were effectively protective oxides, which can prevent the intrusion of oxygen into the alloy substrate. Moreover, a phase transformation (γ/γ′ to γ/Co3W) was observed at the interface between oxide layer and substrate.