<div></div>

div{
   background: #2db34a;
  border-radius: 6px;
  cursor: pointer;
  height: 150px;
  line-height: 95px;
  text-align: center;
  transition-property: border-radius;
  transition-duration: 1s;
  transition-timing-function: linear;
  width: 150px;
}

div:hover {
  border-radius: 50%;
}