The increasing penetration of renewable energy, particularly wind power, raises the need for investigating alternatives to current power system planning and operation approaches due to the uncertain nature of the intermittent generation. This paper proposes a risk assessment approach to evaluate power system security for short-term operation planning under significant penetration of wind power. A composite risk index, which reflects both the probability and consequences of contingencies as well as system uncertainties caused by demand and wind generation forecasting errors, is used to assess the system security. To examine the applicability of the proposed approach, it is applied to a modified IEEE 14-bus system, and compared with traditional deterministic security assessment methods. The results show that the proposed probabilistic approach effectively deals with the system uncertainties, and can provide a more realistic assessment of the system security.