Parallel programming or making parallel application is a great challenging part of computing research. The main goal of parallel programming research is to improve performance of computer applications. A well-structured parallel application can achieve better performance in terms of execution speed over sequential execution on existing and upcoming parallel computer architecture. This thesis named "Evaluations of the parallel extensions in .NET 4.0" describes the experimental evaluation of different parallel application performance with thread-safe data structure and parallel constructions in .NET Framework 4.0. Described different performance issues of this thesis help to make efficient parallel application for better performance. Before describing the experimental evaluation, this thesis describes some methodologies relevant to parallel programming like Parallel computer architecture, Memory architectures, Parallel programming models, decomposition, threading etc. It describes the different APIs in .NET Framework 4.0 and the way of coding for making an efficient parallel application in different situations. It also presents some implementations of different parallel constructs or APIs like Static Multithreading, Using ThreadPool, Task, Parallel.For, Parallel.ForEach, PLINQ etc. The evaluation of parallel application has been done by experimental result evaluation and performance measurements. In most of the cases, the result evaluation shows better performance of parallelism like less execution time and increase CPU uses over traditional sequential execution. In addition parallel loop doesn’t show better performance in case of improper partitioning, oversubscription, improper workloads etc. The discussion about proper partitioning, oversubscription and proper work load balancing will help to make more efficient parallel application. / Program: Magisterutbildning i informatik