<script>
cities = new Object();
cities['河北省']=new Array('石家庄', '张家口', '承德市', '秦皇岛', '唐山');
cities['山西']=new Array('太原', '大同', '朔州', '阳泉');

function set_city(province, city)
{
  var pv, cv;
  var i, ii;
  pv=province.value;
  cv=city.value;
  city.length=1;
  if(pv=='0') return;
  if(typeof(cities[pv])=='undefined') return;

  for(i=0; i<cities[pv].length; i++)
  {
    ii = i+1;
    city.options[ii] = new Option();
    city.options[ii].text = cities[pv][i];
    city.options[ii].value = cities[pv][i];
  }
}
</script>

<form name="form1" action="" method="get">
  <select name="province" onChange="set_city(this, this.form.city);">
    <option value="0">选择省</option>
    <option value="河北省">河北省</option>
    <option value="山西">山西</option>
  </select>
  <select name="city" id="citys">
    <option value="0">选择城市</option>
  </select>
</form>