Hi ,i have a Janome 350e embroidery machine using JEF format,can i use the JEF format on a janome 300e Please.

Yes you can as i have both machines :))

I did a little looking and found these links. I do not have the machine, but I understand it must use a design card. Also, there may be some .jef files the machine will not read.
I hope there is a Cutie that has the machine willing to offer you more help.
Good luck!