Among the main reasons for re-emergence of vaccine preventable diseases were missed or incomplete immunization schedule. The supplementary immunization activity (SIA) is an important intervention done to provide complete immunization coverage among those children. Better outcome came along with good knowledge and perception on the program. Thus, this study aims to assess the level of knowledge and perception of the mothers towards SIA program. A cross-sectional study was conducted among mothers with children ≤ 15 years old in Cheras, Kuala Lumpur. Data was collected by interview using the guided questionnaire consists of four sections to assess the socio demographic, socio economic, knowledge and perception regarding SIA. The questionnaire was validated for internal consistency with Cronbach’s alpha 0.461 for knowledge and 0.729 for perception. A total of 105 respondents with the median age of 33 years (IQR: 28-38) with majority of them are Malays (82.9%), Muslim (83.8%), married (97.1%) and (57.1%) with 1- 2 child in the family. Half of them were from low income family (46.7%) and had secondary education level (54.3%) and were housewives (47.6%). One third of the respondents (33%) never heard about SIA before. Overall had poor knowledge (82.9%) and perception (95.2%) towards SIA. There is a significant association between the level of knowledge on SIA with household income (χ^2=7.746, p=0.019) and occupation (χ^2=6.457, p<0.05). However, there was no significant association between the socio demographic and socio-economic factors and perception towards SIA. In conclusion, the knowledge and perception among mothers on SIA are still poor. More intensive health educations may be useful to be incorporated within the program implementation to increase the community understanding and perception towards SIA in the future.