Mode blocking may be required in mode-division multiplexing (MDM) systems. We demonstrate a silicon mode blocking filter using a subwavelength grating-based contra-directional coupler. The device is capable of blocking the undesired mode channel without affecting the propagation of the other modes. As a proof-of-concept experiment, two mode blocking filters are experimentally demonstrated, which can block the TE0 mode and the TE1 mode, respectively. Low crosstalk (≤21.0 dB) and reasonable insertion losses (≤2.3 dB) are achieved for both the TE0-mode- and the TE1-mode blocking filters.