The state of polarization (SOP) is the key characteristic of light; its evaluation is typically based on measurements of the Stokes parameters. However, conventional approaches often have bulky and complex optical systems with long acquisition times. In this Letter, an ultracompact polarimeter based on dielectric metasurfaces is proposed. The pixel of the proposed metasuface has four areas; each area acts not only as a polarization separator, but also a flat focusing lens. It can acquire a certain polarization component of the incident light and focus the component on the receiver. Therefore, the Stokes parameters of the incident light can be immediately determined through the data from the receiver, fully describing its SOP. The incident waves with different SOPs are simulated to demonstrate that the reconstructed Stokes parameters are in good agreement with the original ones.