<div id="box1" style="width: 100px; height: 100px; background-color: orange"></div> <form> <input type="button" id="btn_start" value="スタート"> </form>

$("#btn_start").on("click", function () { $("#box1").animate({ width: "+=100px", height: "-=50px", opacity: 0.5 }, 1000); });