We have designed and fabricated 2.4mm by 2.4mm 2ch 100MHz/6bits waveform sampling front-end (WSFE) ASIC for PET(Positron Emission Tomography) with multiplexer readouts. This chip was designed for GSO-APD gamma-ray detectors and has the function of "waveform sampling" at the speed of 100MHz.One channel of this chip consists of charge-sensitive preamplifier, VGA (Variable Gain Amplifier), folding-ADC and digital readout circuit and converts the waveform of detector signal to digital values at the speed of ∼100MHz.This digitization in early stage enables flexible digital processing to get timing and energy information of gamma-rays which is indispensable for PET system. Digital circuit has three ways to read signals out, parallel readout, 32words FIFO memory and 2 to 1 multiplexer. The 2 to 1 multiplexer reduces the number of ASIC pins half by using fast clock with double frequency and enables to include more channels in a chip. We designed new 18channel WSFE ASIC which includes 2 to 1 multiplexer, 3 toi multiplexer and 6 to 1 multiplexer. These multiplexers will greatly decrease the number of pins and realize high integration. This chip will be used for APD-based DOI PET system which requires multi-channel ASIC.