Perception of insufficient milk supply (PIM) is the condition in which a mother feels that her breastmilk is insufficient to meet the needs of their babies. Such perception is one of main reasons of the exclusive breastfeeding failure in the world. This study aimed to find out the most dominant factors related to PIM in Tegal Selatan District and Margadana Sub-Districts at Tegal City in 2014. This study was a primary study with a cross sectional design. A total sample of 88 mothers of 0 - 6 months old babies selected in by purposive sampling. Mother’s nutritional status was seen from the increasing of mother’s weight gain during the pregnancy was it met the standards from Institute of Medicine. The mother’s energy intake during lactation was measured through interview using semi-quantitative Food Frequency Questionnaire (FFQ / Food Amount Questionnaire (FAQ)) form. Knowledge variable used structured questionnaire. Analysis used are chi square test (bivariate) and multiple regression logistic (multivariate). The result showed that 51.1% mothers experienced PIM. Variables knowledge (p value = 0.001), energy intake (p value = 0.019), and early initiation of breastfeeding (p value = 0.048) were significantly related to perception after controlled by nutritional status, parity, rooming-in, latch on, family support, and health practitioners support variable. Knowledge is the most dominant factor related to the PIM.