Volume reduction of a radioactive waste solution and recovery of the long-lived radionuclides are very much essential for effective nuclear waste management. A combination of complexation and nanofiltration for selective separation of strontium from a simulated Intermediate-Level Waste (ILW) solution has been studied. The advantages of nanofiltration membranes such as high retention, low operating pressure, and higher permeate flux were explored for the concentrate and contain strategy of waste management. An ethylenimine (EI) oligomer mixture containing primary, secondary, and tertiary amines as functional groups was used as a complexing ligand. The key process parameters, such as the EI concentration, feed pH, and initial strontium concentration, were optimized for strontium rejection. Statistical design of experiments was adopted to study the individual effects and combined interaction effects of these parameters. The optimization of these process parameters for strontium rejection and permeate flux was carried out using central composite design (CCD). The EI concentration and feed pH showed significant but opposing effects on strontium rejection and permeate flux. A strontium rejection of 98% was obtained for an EI concentration of 2.85g/l and a feed pH of 8. The volume of the strontium-containing waste solution was successfully reduced to 10% of the initial volume.