An application of diamond-like carbon (DLC) nano-coated optical fibre as a promising platform for external medium refractive index (RI) sensing is investigated. The DLC thin films are deposited by radio frequency plasma enhanced chemical vapour deposition (RF PECVD) on fused-silica core (Ø = 400 µm) of multimode polymer-clad silica (PCS) optical fibres. The optical fibre sensor response to variations of the external RI for samples coated with DLC film for various thickness and optical properties of the film is measured, and compared to numerical simulations. We show that both optical properties and thickness of the DLC films depend on the deposition time. The effect has a strong impact on the optical response of the samples to variations of the external RI. The response is mainly influenced by the thickness and RI of the DLC overlay. Moreover, the response also depends on the investigated wavelength range. We show that for intensity-based configuration at λ ∼ 490 nm, the DLC film thickness should reach about 80 nm. In such a configuration the sensitivity reaches over 2000% per RI unit (RIU). For wavelength-based configuration, when shift of the intensity deep at λ ∼ 490 nm is considered, the thickness of the film should be as high as 120 nm. In such a configuration the sensitivity reaches 250 nm RIU−1.