The high-temperature strength and compressive creep resistance of TNM B1-type TiAl alloy modified with Y2O3 nanoparticles are studied. The effect of equiaxed or lamellar microstructure on mechanical properties and deformation mechanisms is demonstrated. It is established that the lamellar microstructure provides higher strength compared to the equiaxed one at high temperatures. Microstructural studies of the samples, subjected to compressive creep, revealed that creep is governed by the dislocation mechanisms for the equiaxed alloy, while a combination of dislocation mechanisms, twinning, dynamic recrystallization, and dynamic recovery is typical for the lamellar alloy. In both types of alloys, Y2O3 nanoparticles successfully act as reinforcement at 800 °C, while being prone to failure and facilitating microcrack nucleation at the nanoparticle/matrix interface at 1100 °C.