iam an undergraduate in mechanical engineering, i would like to work on oil and gas sector so i need to learn more about it,im looking for m.sc or pgdip in oil and natural gas engineering
,i have no interest in doing research or phd in future.so will pgdip will be enough for me to get a better chance in the job or does employers (oil companies) prefer one with m.sc

I'm not sure of the diference - can anyone help?

A PGDip tends to be the taught part of the MSc without the research project. There is a perception problem as it is often awarded when someone has failed the MSc overall but passed sufficient taught credits to receive a PGDip. Therefore my gut reaction would be that you would be better doing a full MSc unless you know that in this specific sector, the qualification is not viewed with scepticism.

I can only comment on psychology fields given my background. For some careers (ie professional psychology) a full MSc (with at least a merit) is required to complete professional training. Also, some graduate jobs (within psychology) require applicants with completed relevant MSc qualifications. Not sure if engineering implements similar requirements.

I hope others don't consider my PGCert (with a high merit) a failed MSc! No intentions of completing another MSc! :)