Pursuit of the lowest size-limit of wireless sensor nodes may not only reduce power consumption and production cost, but also enables its layout-free ubiquitous applications, i.e., in our green sensor networks to compress energy consumption through visibility and optimization. In this paper, we engaged in developing the world smallest wireless sensor node with ultralow power consumption from both electrical block integration and physical interconnection points-of-view. A customized IC for signal processing and data transmission, which has universal interface to sensors and power management capability, was designed and then fabricated by using 0.18 μm 1.8 V/3.3 V 1P6M logic process. By introducing buried bump interconnection technology, we have successfully obtained one of the world's smallest wireless sensor nodes, as small as 3.9 mm × 3.9 mm × 3.5 mm, for humidity and temperature monitoring. The sensor node also features an general purpose interface, available for analog and digital sensors, and the ultrasmall footprint of above sensor node enables its layout-free distribution or integration inside other remote sensing systems. Although the adopted antenna size was 2 cm × 5 cm, its flexibility enables free attach to any curved surfaces. Experimental results demonstrated that besides preferred ultralow power consumption and data transmission distance, configuration of above sensor nodes enables its easy assembly with stand-alone power source and flexible antenna for wide variety of applications.