/// <summary>
/// DataTable to JSON converter
/// </summary>
public class JavaScriptDataTableConverter : JavaScriptConverter {
 public override object Deserialize( IDictionary<string, object> dictionary, Type type, JavaScriptSerializer serializer ) {
  throw new NotImplementedException( "Deserialize is not implemented." );
 }

 public override IDictionary<string, object> Serialize( object obj, JavaScriptSerializer serializer ) {
  DataTable dt = obj as DataTable;
  Dictionary<string, object> result = new Dictionary<string, object>();

  if( dt != null && dt.Rows.Count > 0 ) {
   // List for row values
   List<object> rowValues = new List<object>();

   foreach( DataRow dr in dt.Rows ) {
    // Dictionary for col name / col value
    Dictionary<string, object> colValues = new Dictionary<string, object>();

    foreach( DataColumn dc in dt.Columns ) {
     colValues.Add( dc.ColumnName, // col name
      ( string.Empty == dr[dc].ToString() ) ? null : dr[dc] ); // col value
    }

    // Add values to row
    rowValues.Add( colValues );
   }

   // Add rows to serialized object
   result["rows"] = rowValues;
  }

  return result;
 }

 public override IEnumerable<Type> SupportedTypes {
  //Define the DataTable as a supported type.
  get {
   return new System.Collections.ObjectModel.ReadOnlyCollection<Type>(
    new List<Type>(
     new Type[] { typeof( DataTable ) }
    )
   );
  }
 }
}

using System.Web.Script.Serialization;

// ...

[WebMethod()]
[ScriptMethod( ResponseFormat = ResponseFormat.Json )]
public string TestJS( int Id ) {
 JavaScriptSerializer toJSON = new JavaScriptSerializer();
 toJSON.RegisterConverters( new JavaScriptConverter[] { new JavaScriptDataTableConverter() } );

 DataTable dt = new Query( Log.Schema )
   .WHERE( Log.Columns.ID, Id )
   .ExecuteDataSet().Tables[0];

 return toJSON.Serialize( dt );
}