Your code works fine.
R1=5;
R2=7;
thetaP=linspace(0, 2*pi);
plot(R1*cos(thetaP), R1*sin(thetaP), '-b', ...
R2*cos(thetaP), R2*sin(thetaP), '-r')
Possibly, figure is being off. Please try adding figure before plot.
figure;
plot(R1*cos(thetaP), R1*sin(thetaP), '-b', ...
R2*cos(thetaP), R2*sin(thetaP), '-r')