Thanks a lot.I will have a try

class Simplecircuit1

import Modelica;
Resistor R1(R=10);
Capacitor C(C = 0.01);
Resistor R2(R2=100);
Inductor L(L=0.1);
VsourceAC AC;
Ground G;
equation
connect(AC.p,R1.P);
connect(R1.n,C.p);
connect(C.n,AC.n);
connect(R1.p,R2.p);
connect(R2.n,L.p);
connect(L.n,C.n);
connect(AC.n,G.n);
end Simplecircuit1;

Error infoemation: [Simplecircuit1: 10:1-10:9]: Class Ground not found in scope Simplecircuit1.
So how should I deal with this error?