threejs how to rotate around object's own center,instead of world center

If your mesh is not rotating around its center, it is because the geometry vertices are offset from the origin.

You can automate repositioning by using a bounding box to define a reasonable center, and then offset the mesh's position like so:

var box = new THREE.Box3().setFromObject( mesh );
box.center( mesh.position ); // this re-sets the mesh position
mesh.position.multiplyScalar( - 1 );

Then add the mesh to a pivot object:

var pivot = new THREE.Group();
scene.add( pivot );
pivot.add( mesh );

In your animation loop, rotate the pivot;

pivot.rotation.y += 0.01;

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EDIT: A different solution is to translate the geometry vertices so the geometry is centered around, or near, the origin:

geometry.translate( distX, distY, distZ );

Or, alternatively, you could just call:

geometry.center();

which centers the vertices of the geometry for you based on the geometry's bounding box.

three.js r.97

Use THREE.Geometry.prototype.center as follows:

myGeometry.center();

This is like using myGeometery.translate(x,y,z) with automatic centering (x,y,z).

The pivot solution was not working for me.

With OBJLoader.js (for loading .obj objects), you need to get the boundingBox of the object, get its center, multiply scalar by -1, and use this to translate the geometry of EVERY child's geometry. Then, you need to reUpdate the boundingBox if you want to use it for a purpose.

Here is a function that loads an obj and "normalize" its geometry vertexes, given the directory and name of the OBJ and MTL (texture) files (for example, if the OBJ and MTL files are dir1/myObject.obj and dir1/myObject.mtl, then you call loadObj('dir1','myObject')).

 function loadObj(dir, objName) {
        var onProgress = function(xhr) {
            if (xhr.lengthComputable) {
                var percentComplete = xhr.loaded / xhr.total * 100;
                console.log(Math.round(percentComplete, 2) + '% downloaded');
            }
        };

        var onError = function(xhr) {};

        // Manager
        var manager = new THREE.LoadingManager();
        manager.onProgress = function(item, loaded, total) {
            console.log( 'Started loading file: ' + item + '.\nLoaded ' + loaded + ' of ' + total + ' files.' );
        };

        var mtlLoader = new THREE.MTLLoader();
        mtlLoader.setPath(dir);
        mtlLoader.load(objName + '.mtl', function(materials) {
            materials.preload();

            // Model
            var loader = new THREE.OBJLoader(manager);
            loader.setMaterials(materials);
            loader.setPath(dir);
            loader.load(objName + '.obj', function (object) {

            var objBbox = new THREE.Box3().setFromObject(object);

            // Geometry vertices centering to world axis
            var bboxCenter = objBbox.getCenter().clone();
            bboxCenter.multiplyScalar(-1);

            object.traverse(function (child) {
                if (child instanceof THREE.Mesh) {
                    child.geometry.translate(bboxCenter.x, bboxCenter.y, bboxCenter.z);
                }
            });

            objBbox.setFromObject(object); // Update the bounding box

            scene.add(object);
            }, onProgress, onError);
        });
    }