Hello Graynomad, thanks for your help 
I have changed my program as you suggested. I have included a null terminator and used strcmp but still the output is same 
Do you see any logical error in my code?
int ledPin = 13;
const int maxLength=140; //maximum length for textbox data
char char_in[maxLength]; //Create a buffer to input all incoming data
int i=0;
void setup() {
pinMode(ledPin, OUTPUT); //Configure pins as output
Serial.begin(9600); //begin serial communication
}
void loop() {
if (Serial.available() > 0) //Is any serial data available
{
for( i=0;i<maxLength && Serial.available();i++)
{
char_in[i] = Serial.read();
}
char_in[i+1]='\0';
if (!strcmp(char_in,"on1"))
{
digitalWrite(ledPin,HIGH);
}
else if(!strcmp(char_in,"of1"))
{
digitalWrite(ledPin,LOW);
}
else
{
Serial.print(char_in);
}
}
}