Organ size control is a fundamental developmental processes for higher plants as well as a promising target trait for molecular breeding in crop plants. Genetic mechanisms how plant organs grow to a certain size remains still unclear. Here we present the identification and characterization of a genetic mutant, big flower1-1 (bif1-1) in Arabidopsis that exhibits bigger organ size primarily due to increased cell size. Genetic analysis indicated that it is a single, semi-dominant mutation. Phenotypic analysis showed that bif1-1 exerts pleiotropic effects: it caused bigger seed size, bigger seedling, bigger leaf, thicker stem, increased trichome branching, smaller fruit, and bigger pollen. Microscopic analysis suggested that the bigger organ size in bif1-1 mutant is primarily attributed to increased cell size. Gene expression analysis indicated that most of growth-control genes tested were not altered in bif1-1 mutant. Instead, expression of ARGOS and auxin-inducibility of ANT were reduced in bif1-1 mutant. Our ongoing positional on the corresponding gene would not only shed light on the molecular mechanisms how plants adopt final organ size but also provide a promising genetic resource for genetic engineering of flower- and seed-size in crop plants.