Increasing number of iterations not increasing time
The compiler has realized that your loop doesn't do anything useful and therefore optimized the whole thing away. Thus you are just timing how long it takes to do nothing. (You didn't get a result of zero, because calling micros will itself take time).

BTW, the granularity of micros is 4 µs, so you won't get a reading of 1 or 2. The code actually probably only took a couple of microseconds.

You need to do something that the compiler thinks "does something", like this:

void setup() {
    Serial.begin(115200);
}

volatile byte b;

void loop() {
    unsigned long a = micros();
    for (unsigned long j = 0; j &lt; 100; ++j)
      b++;
    Serial.println ((micros() - a));
    delay(1000);
}


By making b volatile the compiler cannot decide that b isn't doing anything useful, so now the timings make sense.

I got 72 for 100 iterations and 144 for 200 iterations.




  Is there a function which I can use to measure time properly in arduino?


Your time measurement is fine. Your interpretation of the results indicates you didn't realize how smart the compiler is. :)



You can see what the compiler did if you examine the generated code:

    unsigned long a = micros();
  d8:   0e 94 d2 00     call    0x1a4   ; 0x1a4 &lt;micros&gt;
  dc:   6b 01           movw    r12, r22
  de:   7c 01           movw    r14, r24
    for (unsigned long j = 0; j &lt; 100; ++j);
    Serial.println ((micros() - a));
  e0:   0e 94 d2 00     call    0x1a4   ; 0x1a4 &lt;micros&gt;
  e4:   ab 01           movw    r20, r22
  e6:   bc 01           movw    r22, r24


As you can see, between the two calls to micros there isn't any sort of loop.

To find the assembled code you can follow the steps documented here.

Basically, turn on "verbose compiling" which will show near the end of the verbose output the full pathname of the ".elf" file. Then use a command (terminal) window to enter this command:

avr-objdump -S xxx.elf


You may want to redirect the output to a file:

avr-objdump -S xxx.elf &gt; myfile.txt


Depending on how the Arduino IDE was installed you may have to find the pull pathname of avr-objdump and use that instead of just the program name.