In situ whisker growth was observed during heat treatment of allylhydridopolycarbosilane (AHPCS) and SLC powder in the temperature range of 1250 degrees -1350 degreesC. TEM equipped with an electron energy loss spectrometer (EELS) verified that the banded whiskers are twinned single-grained P-SIC. Convergent beam electron diffraction patterns (CBED) of the whisker tips were consistent with formation by a vapor-solid (VS) mechanism. The effects of process variables on whisker growth were addressed. The mechanism of whisker growth was discussed and attributed to the reaction between the gaseous products of the polymers AHPCS and polycarbosilane (PCS). Thermal decomposition behavior of the polymers was followed to relate gas evolution to whisker formation.