This paper reports a method for measuring the heat capacity of as-grown carbon nanotubes (CNTs) using a microcalorimeter. The microcalorimeter consists of a double-layer suspended silicon dioxide microstructure and two silicon resistors sandwiched in-between the silicon dioxide layers. CNTs for heat capacity measurement are locally synthesized on the surface of the microstructure using laser-assisted chemical vapor deposition. The CNTs and the microcalorimeter are heated to a high temperature with a silicon resistor, and the temperature of the microcalorimeter with CNTs is measured with using the other silicon resistor. The heat capacity of the CNTs is obtained by dividing the measured effective heating power by the rate of temperature increase. The suspended configuration and the low thermal conductivity of silicon dioxide achieve low thermal dissipation, which together with the minute thermal capacity enable significant temperature changes. Using this sensitive microcalorimeter, the heat capacity of as-grown CNTs, 6-14 nJ/K from 340 to 440 K, is measured in-situ without the need of CNT transfer and pretreatment, avoiding damage to the CNT samples. This microcalorimeter is also applicable to in-situ measurement of structure-related thermal properties of porous materials.