Abstract Influence of isothermal aging on the microstructures and hardness in Inconel 740, a relatively new Ni-based superalloy, was explored using the specimens aged at 810 °C for different times. As aging time increased, the size of gamma prime precipitates continuously increased while their fraction remained almost constant. Nanoindentation experiments revealed that the overall hardness increased till the aging time of 100 h and then decreased with the aging time. Estimation of phase hardness by applying a simple rule-of-mixture showed that, with aging time, the hardness of gamma matrix decreased whereas that of gamma prime precipitates increased. The aging-induced strength change is discussed in terms of the possible contributions of precipitation strengthening and solid solution strengthening.