使用PerformanceCounter：

        using System.Diagnostics;

        public float GetCpuUsage()
        {
            var cpuCounter = new PerformanceCounter
            {
                CategoryName = "Processor",
                CounterName = "% Processor Time",
                InstanceName = "_Total"
            };
            return cpuCounter.NextValue();
        }

使用WMI：

using System;
using System.Management;
using System.Windows.Forms;

namespace WMISample
{
  public class MyWMIQuery
  {
    public static void Main()
    {
        try
        {
            ManagementObjectSearcher searcher = 
                new ManagementObjectSearcher("root\\CIMV2", 
                "SELECT * FROM Win32_PerfFormattedData_PerfProc_Process"); 

            foreach (ManagementObject queryObj in searcher.Get())
            {
                Console.WriteLine("-----------------------------------");
                Console.WriteLine("Name: {0}", queryObj["Name"]);
                Console.WriteLine("PercentProcessorTime: {0}", queryObj["PercentProcessorTime"]);
            }
        }
        catch (ManagementException e)
        {
            MessageBox.Show("An error occurred while querying for WMI data: " + e.Message);
        }
    }
  }
}

// 引用来源 http://stackoverflow.com/questions/22195277/get-the-cpu-usage-of-each-process-from-wmi