An experiment was carried out to study the effect of planting space, some weather factors and plant age on the population density of Leaf miner, Liriomyza trifolii (Burgess) infesting pea plants in Plant Protection Research Station in Kaha region during two successive seasons 2013 and 2014. The obtained results revealed that the population density of L. trifolii larvae on pea plants differed significantly according to the planting spaces (10cm, 20cm and 30cm) during the two studied seasons. Planting of pea seeds in the planting space (10cm) lead to plants suffered from the highest infestation by L. trifolii. Larvae (30.7 and 34.1 larvae/ 15 leaves during the two seasons, respectively. On the contrary, sowing the pea plants at the second and third spaces (20 & 30cm) can be reduced the infestation rate of this insect pest (4.97 & 3.2 larvae/ 15 leaves) and (5.16 & 2.6 larvae/ 15 leaves) during the two seasons, respectively. The population density of leaf miner, L. trifolii increased by increasing the plant age until middle of the flowering stage. The lightest population was recorded during the fruiting stage. While, the pea plants infested by the heaviest population during vegetative and flowering and stages. Statistical analysis of correlation coefficient and  partial regression values showed that the effect of weather factors  and plant age on the population density of  L. trifolii was differed to positive or negative correlation according to planting spaces and plant stages (vegetative, flowering and fruiting stages) during two studied seasons. The weather factors were more effective on leaf miner population when comparing with plant age, the explained variance (E.V. %) were 74.4, 66.4 and 72.3% for the three planting spaces in the first season and 18.5, 29.3 and 40.4% in the second season, respectively. While the combined effect of  the plant stages (vegetative, flowering and fruiting stages) together on L. trifolii population in the tested planting spaces on pea plants were 38.5, 21.1 and 15.7 at the three planting spaces during 2013 season and 27, 10.2 and 6.3 % during 2014 season, respectively.