Posting value to web api method from ajax call why going null?

I am posting some values to web API method when I debug it's showing null values.

Below is a snipet code of ajax call, here I am passing dept_name but it's showing null in web API method.

var _customer = {};
            _customer.DepartmntName = txtName.val();
            _customer.DateCreated = null;
            _customer.DateModified = null;
            _customer.IsDeleted = null;
            _customer.ParentID = null;
            _customer.Icon = null;
            _customer.Paren = null;
            alert( _customer.DepartmntName);
            alert(_customer);
            $.ajax({
                type: "POST",
                url: "/api/AddDepSpc/InsertCustomer2",
                 data: JSON.stringify(_customer),
                contentType: "application/json",
                dataType: "json",
                success: function (r) {
                alert("success" + r);
               var row = $("#tblCustomers tr:last-child").clone(true);
                    //AppendRow(row, r.CustomerId, r.Name, r.Country);
      AppendRow(row,r.dept_name.r.date_created,r.date_modified,r.IsDeleted,
       r.ParentID.r.Icon,r.Paren);
                    txtName.val("");

                }
            });

Below snippet code for web api method.

public class AddDepSpcController : ApiController
    {
        [Route("api/AddDepSpc/InsertCustomer2")]
        [HttpPost]
        public   master_department InsertCustomer2(master_department _customer)
        {           
            using (HRMEntities entities = new HRMEntities())
            {
                entities.master_department.Add(_customer);
                entities.SaveChanges();
                entities.SaveChanges();
            }
            return _customer;
        }      
    }

Now in debug mode it shows null supposed I have passed HR as department name and all the rest of parameter are null now.Expecting HR parameter value in web api method.

Your posting back an object that contains property names that do not match your model.

You model contains a property named dept_name, but the javascript object contains one named DepartmntName. Similarly, none of the other names match (except Icon). Assuming you want to bind the value of txtName to the models dept_name property, then in the script, use

var _customer = {};
_customer.dept_name = txtName.val(); // not DepartmntName
....

Note also that there is no reason to add the null values to the _customer object (they will be null by default in the controller). You can also delete the contentType: "application/json", option and just use data: _customer, in the ajax call instead of stringifying it.