JSON_PARSER_EXCEPTION "Unexpected parameter encountered during deserialization:

So, what's going on is that this statement

global static void CreateOrderReq(cls_DummyRequest cObj) 

is going to automatically deserialize the JSON into cObj per the type cls_DumyRequest

But the incoming JSON is:

{ "DummyRequest":{ "HeaderData":{ "RecordTypeName":"abc" }, "Cpdt":[ { "Item":"48262" } ], "CasoDt":{ "CaseObj":"" } } } 

Per the SFDC Apex REST doc , when you use the implicit deserialization argument, the top level property of your incoming JSON object must agree with the argument name

Since the top level property for your JSON is DummyRequest, then the POST method's argument must be named the same:

global static void CreateOrderReq(cls_DummyRequest DummyRequest) 

and you should remove the declaration in the class:

global cls_DummyRequest DummyRequest; // remove this line 

this leaves you with:

@RestResource(urlMapping='/CaseRecCreation/*') global without sharing class APICreatRec{ global class cls_DummyRequest { cls_HeaderData HeaderData; cls_Cpdt[] Cpdt; cls_CasoDt CasoDt; } class cls_HeaderData { String RecordTypeName; } class cls_Cpdt { String Item; } class cls_CasoDt { String CaseObj; } @HttpPost global static void CreateOrderReq(cls_DummyRequest DummyRequest) { // local variable DummyRequest has the deserialized JSON Case orc= new Case(); } } 

Note only the implicit deserialization class cls_DummyRequest need be declared global. Everything else can be private. I tested this in my Trailhead org.