During transformation in the laboratory the DNA does not destroyed,even it is not methylated.why?

Depends on the stain being transformed. Read your protocol.

I'm assuming you're talking about transformation of bacteria colonies and inducing them to take up a DNA fragment. The point of this is to maintain and amplify the DNA, so obviously the protocol won't include any conditions that will destroy the DNA. Usually the DNA used is also a fragment that was synthesized in vitro, with PCR or the like, so the DNA won't be methylated - but I would assume that after transformation, when the bacteria replicate, that the DNA within the bacteria would then become methylated, since it is being synthesized in vivo. I'm not entirely sure about that, though.

The OP refers to methylation, so clearly it meant degradation of DNA after uptake. Also, transformation is usually done with a vector, which is propagated in vivo. However, even those can be unmethylated. As I said before, it depends on the strains.

Thanks to Paralith and Charnoy! But I have a confusion that the bacterial restriction enzyme does not recognize its own DNA to cut coz the restriction site become methylated but if we incorporate the restriction enzyme into the genome of other bacterium it will recognize n cut the site coz there is no methylation,so when in the labs when we transfer the DNA from one strain to another,then why it is not destroyed by the restriction enzyme of the recipient strain.

You have to read up on types of restriction/modification enzymes, the genes encoding them and then check the genotypes of the strains used for transformation.