A method and apparatus for performing transmission in a wireless communication system is provided. A narrowband internet-of-things (NB-IoT) user equipment (UE) transmits a physical uplink shared channel (PUSCH) to a network by using a first resource unit, and transmits an acknowledgement/non-acknowledgement (ACK/NACK) to the network by using a second resource unit. The first resource unit for PUSCH transmission consists of a first number of resource elements (REs) within a first tone in frequency domain and a first time interval in time domain. The second resource unit for ACK/NACK transmission also consists of a second number of REs within a second tone in frequency domain and a second time interval in time domain. The second number is smaller than the first number.