In this paper, we present two different designs for Ultra High Frequency (UHF) Radio Frequency Identification (RFID) passive tags. The first proposed design has been mounted on AK3 Tagsys^® loop, which is a small loop antenna already connected to the chip while the second approach is connected directly with RFID chip. The optimized designs have compact size and bandwidth for worldwide UHF RFID applications (860MHz to 960 MHz). The advantages and disadvantages of each approach will be also discussed. From the experimental results, these designs have the acceptable read-range for ETSI regulations of RFID (9.6m and 5.97m) and can be applied for real-world applications. However, the calibration between the simulation and the characterization in order to remove the tolerance of fabrication process should be executed in future steps to obtain the best performance by improving the proposed designs.