The benefits of circumferential groove casing treatment on the performance of a low-speed contra-rotating fan are investigated. Three-dimensional, time-dependent simulations are carried out with the k-ω SST-SAS hybrid turbulence model using the open-source CFD library OpenFOAM. The numerical model is validated with experimental data from the contra-rotating fan with a smooth casing. This comparison showed a very good agreement. Then, two casing treatment variations are analyzed: 1) circumferential grooves on top of the front rotor, and 2) circumferential grooves on top of the rear rotor. Simulating the performance curve at design speed reveals an increase in pressure rise for casing treatment at the front rotor of up to 4 %. This results from significantly reduced blade pressure fluctuations and weakened blade tip vortices at the front rotor. A weaker tip leakage vortex leads to a less disturbed inflow for the rear rotor and thus pressure fluctuations. In contrast, grooves on top of the rear rotor offer no positive effect since there is no rotor downstream to benefit from reduced fluctuations or weakened tip leakage vortex. Pressure probes downstream of the rear rotor were evaluated using FFT. Grooves reduce the magnitude of blade passing frequencies and their harmonics while partly increasing lower frequencies.