Nanoscale thin film transistors with conjugated polymers as the active layer were fabricated to investigate their chemical sensing properties. Guarding electrodes as close as 20 nm to the two sides of the channel were employed to eliminate undesirable spreading currents and ensure that the sensor active area is truly nanoscale. The response of drain current exhibited opposite directions in nanoscale sensors compared to large scale devices, for the same analyte-semiconductor combination. The transition in response behavior was observed occurring in a certain interval of channel length. The chemical sensing mechanisms in both microscale and nanoscale transistors are briefly discussed.