We can hypothesed by comparing it with other bands, the band which has less thickness as well as less intensity of EtBr flouroscent.

Plasmid will have discreet band on EtBr stained agarose gel. Chromosomal DNA will be visualized as high molecular weight smears.

First of all if you are isolating plamid DNA from culture then there are different isolation method for plasmid. In isolation of plasmid method you will get only plasmid and not genomic DNA. And if you are starting with DNA sample then run the sample in low concentraion of gel. the band which has moved a long distance is your plasmid DNA.

Do you have a gel picture for us to have a look at?

Maybe this picture may help: http://humgen.wustl.edu/hdk_lab_manual/image/plsmid3.gif

Thanks to all for helping me .

@ Manojkumar Avaghade

Can you please share different isolation method for plasmid, I am interested too. . .

thanks