You need to have a reference of the first form in the second form in order to access the variable in the first form.

Something like this:

Form1 oFrm1 = new Form1();
Form2 oFrm2 = new Form2();

oFrm2.ReferenceToForm1 = oFrm1;


Then you can access the public variable of Form1 from Form2 by this.ReferenceToForm1.PublicVariable