Superconducting nano-electronic devices are appealing candidates for implementing quantum-bits1 (qubit) because they can be fabricated in parallel using lithography techniques. At the present time, the most advanced qubits are the single Cooper pair box2 (CPB), which involves charge states of a small superconducting island, the RF-SQUID3 and the 3-junction SQUID4. Recently, Nakamura5 et al. demonstrated temporal coherence of quantum superpositions of qubit states in the CPB. The decay of Cooper pairs into quasiparticles used in this experiment for measuring the CPB is far from providing a single shot readout, and severely limits the coherence time of the superposition of states. Achieving single shot determination of the qubit energy eigenstate while preserving the coherence prior to measurement is a central issue. Recently, fast electrometers6,7,8,9 have been developed for measuring the charge of the CPB island, and the sensitivity necessary for such single-shot readout has already been reached by an electrometer based on the radio-frequency Single Electron Transistor. In this work, we present a new readout scheme based on the single Cooper Pair Transistor10,11,12 (CPT). In this superconducting device, the supercurrent is modulated by the charge coupled to the transistor island. We have measured the charge resolution of such an electrometer, and evaluated the back-action it would have on a CPB.