The research aims to（1）study the behavior of Gen Y Thai consumers and digital marketing of LAZADA in Thailand,and（2）find the most important factors of digital marketing affecting Thai consumers on e-commerce via LAZADA.The sampling group is ecommerce users on LAZADA,GEN Y about 400 people.The research instrument is a questionnaire.Statistics used for the research are frequency,percentage,mean,standard deviation,and hypothesis testing in regression.The results are as follows.1)The behavior of Gen Y Thai consumers and digital marketing of LAZADA in Thailand;the reason they purchase goods or use services via LAZADA due to relatively lower than the general market-based on 143 people or 35.80%,followed by oneself as an influence of consuming goods and services on LAZADA made of 278 people or 69.50%.About 182 people or 45.50% has a frequency of consumption via LAZADA of 1-2 times per month.The majority of the respondents purchase goods or services on LAZADA at the price between 501-1,200 THB which is made of 172 people or 43.00%.The most used device to enter the LAZADA website is through Google with made of 349 people or 867.30%.About154 people or 38.50% spends time browsing the website between 16.01-20.00 and 20.01-24.00 equally.And the most frequent category of purchasing goods or services is health and beauty which made of 193 people or 33.30%.2.The most important factors of digital marketing affecting Thai consumers on ecommerce via LAZADA include business-to-business（B2B）,business-to-consumer（B2C）,and consumer-to-consumer（C2C）,respectively.