Facing problems measureing Input offset voltage of opamp
Your circuit is not ideal. The relatively high 1K || 100K resistance the inverting input "sees" will lead to an additional term dependent on the bias current of that input. It would be better to use a much lower resistance (minimizing the effect of the input offset current) and to add a similar resistance to the non-inverting input (minimizing the effect of the two bias currents). 

Here is a typical circuit. You should have bypass capacitors (10n to 100n ceramic is fine) near the op-amp power pins. Not really vital for very slow op-amps, but some day you may wish to test a fast op-amp. In such a case you may wish to also parallel R1 with a 10n NP0 capacitor to tame any oscillation. You may not be able to measure worst-case offset with low voltage supplies so you may have to lower the gain by lowering R1/R3 to 4.99K or increasing R2/R4 to 20 ohms. 







simulate this circuit &ndash; Schematic created using CircuitLab

The datasheet shows the typical/maximum (under specific conditions) magnitude of Vos. Vos can be of either polarity- it's just as likely to be positive as negative for most op-amps (the target value is zero). 

It's (barely) possible the particular sample of LM358 you have happens to have very low offset voltage. Or maybe (perhaps more likely) you have it hooked up wrong- the pinout is different, of course, being a dual.