Since 1995, the Cambodian Rubber Research Institute (CRRI) has established a network of clone trials in the rubber growing traditional regions in Kampong Cham, Kratie and Rattanakiri Provinces in an altitude lower than 200 m.a.s.l. The ecological conditions were noted that 6-month rainy season and 6-month drying season, which the absolute temperature is in the interval of 16 °C and 410 C, the wind speed varied from 1 m/s to 7 m/s and the average rain fall is lower than 1,500 mm. The Agro-climatic constraint for rubber growing in Cambodia is the rainy season including an irregular drying season, which causes a big problem for planting rubber trees. The most important rubber diseases in Cambodia are Pink disease and Oidium leaf fall disease. Colletotrichum leaf disease, Phytophthora panel disease and Phytophthora leaf fall disease are sometimes severe, others diseases are not of importance. Root diseases and Corynespora are temporarily absent in Cambodia. Up to now, there are 19 LSCT and 20 simple small scale clone trials (SSSCT) have been established. CRRI has established and continuously surveyed a network of clone trials in major rubber growing areas. The result derived from network clone trials permitted to upgrade the promising clones for the planting recommendation, which has been improved every 3 years. Database from the network of LSCT planted in 1995-96 indicated that, under the favorable conditions of central, East and North-East regions, the growth in immature period to the first four years of tapping of clones PB235, IRCA130, PB330, PB280, RRIC101, RRIC110, IRCAllland KV4 appeared to be better than GT1 (control clone). Clones KV4, PB260, RRIM600 and PR107 had moderate growth and comparable to GT1 and the growth of IRCA18, RRIM712 and PR series clones appeared lower than GT1. The LSCT planted in 1986 in CHUP company shows that the cumulated yield in 12 years of clones KV4,'PB235, RRIM600, PB324, PB310, PB255, RRIC110, RRIC121, PB255 and PB324 are higher tha