  We present a theory describing the lambda transition and the Bose-Einstein
condensation (BEC) in a liquid ${\rm ^4He}$ based on the diatomic quasiparticle
concept. It is shown that in liquid ${\rm ^4He}$ for the temperature region
$1~{\rm K}\leq{\rm T}\leq {\rm T}_\lambda$ the diatomic quasiparticles
macroscopically populate the ground state which leads to BEC in liquid ${\rm
^4He}$. The approach yields the lambda transition temperature as ${\rm
T}_\lambda=2.16~{\rm K}$ which is in excellent agreement with the experimental
lambda temperature ${\rm T}_\lambda=2.17~{\rm K}$. The concept of diatomic
quasiparticles also leads to superfluid and BEC fractions which are in a good
agreement with the experimental data and Monte Carlo simulations.
