I will take a stab at this...
I am going to assume vickyjaju is referring to the Pentium Dual-Core CPU series which is based on the Allendale / Wolfdale cores used in the Core 2 Duo series.
The Pentium Dual-Core CPUs are slower than Core 2 Duo CPUs because they use slower RAM. The Allendale Pentium Dual-Core CPUs uses DDR2-400 RAM, Allendale Core 2 Duo CPUs uses DDR2-533 RAM. Wolfdale Pentium Dual-Core CPUs can use up to DDR2-533 RAM, while Woldale Core 2 Duo CPUs can use up to DDR2-667 RAM.
There is also a difference in the L2 cache. The Allendale Pentium Dual-Core CPUs has 1MB of L2 cache, Allendale Core 2 Duo CPUs has 2MB or 4MB of L2 cache pending on the model. Wolfdale Pentium Dual-Core CPUs also have 2MB of L2 cache, while Woldale Core 2 Duo CPUs have 3MB or 6MB of L2 cache depending on the model.
Overall, Pentium Dual-Core CPUs are probably around 5% to at most 10% slower than Core 2 Duo CPUs.