If you are using Bootstrap 3.3.x then use this code (you need to add class name carousel-fade to your carousel).
.carousel-fade .carousel-inner .item {
-webkit-transition-property: opacity;
transition-property: opacity;
}
.carousel-fade .carousel-inner .item,
.carousel-fade .carousel-inner .active.left,
.carousel-fade .carousel-inner .active.right {
opacity: 0;
}
.carousel-fade .carousel-inner .active,
.carousel-fade .carousel-inner .next.left,
.carousel-fade .carousel-inner .prev.right {
opacity: 1;
}
.carousel-fade .carousel-inner .next,
.carousel-fade .carousel-inner .prev,
.carousel-fade .carousel-inner .active.left,
.carousel-fade .carousel-inner .active.right {
left: 0;
-webkit-transform: translate3d(0, 0, 0);
transform: translate3d(0, 0, 0);
}
.carousel-fade .carousel-control {
z-index: 2;
}